Gillard told it herself, to an ABC reporter in 2006, well before she had to be on better behaviour as a PM.
She was health spokeswoman and was in a relationship with Craig Emerson, now Trade Minister, a fellow MP who had entered federal politics with her in 1998.
"Craig and I were staying together at a hotel and I'd managed to forget to pack my contact lens holder. So I was just storing the contact lenses at the bottom of a glass, which wasn't exactly the smartest thing in the world to do. So ... in the bathroom, [there is] this glass with the contact lenses and a bit of solution in them. So, you know, during the course of the night, Craig gets up and thinking it's water, grabs the glass and drinks it.
"So I was wandering around [the] national conference blind for the next morning. I did have to give the health policy report at the podium not basically able to see my notes or the audience."
If the story tells us anything about her, it is that she has a decent sense of humour. She has never confirmed that she was "'the other woman" but he left his wife and three children about the same time.
Gillard's visit to Queenstown this weekend for the annual summit with the New Zealand Prime Minister may be a welcome distraction from her difficult start to the political year.
Despite mounting a failed comeback bid last year, Kevin Rudd is still the monkey on her back, proclaiming his loyalty while digging in his claws a little deeper. His predecessor, Mark Latham, this week described him as the "worst saboteur in the history of the Australian Labor Party".
Latham also believes the Canberra press gallery is displaying a "disgraceful bias" against Gillard and even if she cured cancer and climate change, they would still be on her back.
She is commonly described as being disliked by voters but consistently polls higher than the Opposition alternative, Tony Abbott, as preferred Prime Minister.
Whether the knocking from her critics is fair or not, Gillard's private and public life has provided enough stories for forests' worth of newsprint.
Gillard and Emerson lasted two years. Her take on the break-up is that with her being based in Melbourne and him and Queensland, not to mention the hurly burly of politics, it was impossible to maintain the relationship.
Some media suggested that it was their ambitions that were incompatible, not their timetables.
Gillard's partner is now Tim Mathieson, and she has brought him to Queenstown. He is a former hairdresser and her easy-going de facto husband of seven years with adult children. He's of a different cut to her earlier relationships with the union men she associated with as a former union lawyer.
Some feared that Mathieson's lack of political grounding might make him a liability as Australia's First Bloke, but he has proved not to be.
Well, almost not.
While making a speech last month to encourage other men to have a physical examination for prostate cancer, he joked that perhaps they should look for a small female Asian doctor to conduct it. Gillard was standing next to him and the cameras captured her unamused glance at him - he apologised the next day for any offence his joke had caused.
In many quarters, the laugh was seeing Gillard squirm, especially after an electrifying speech she gave on sexism in Parliament at the end of last year that went viral.
She was pilloried by many political commentators for the speech, but it went down a treat with much of the public in Australia, and all over the world. Public sympathy towards her was at an all-time high because her father had just died when word got out that radio jock Alan Jones had suggested at a Liberal Party dinner that he had died of shame over her.
Gillard unleashed a withering attack on Liberal leader Tony Abbott in a deeply personal unscripted speech about what she described as his sexist and misogynist record which she itemised over 15 minutes.
He appeared shocked at the vehemence of the attack. For those 15 minutes she dispensed with her taut style, and Gillard unplugged showed some passion.
Rod Emmerson, the Herald's cartoonist and an Australian immigrant, believes Gillard was thrown into the leadership and struggles because she is out of her depth. It doesn't help that she leads the Labor Party - "a huge dysfunctional family".
He is impressed that she shows her humanity to the world but believes she is going to lose the next election "in a big way".
Gillard is the daughter of working class, Labor-supporting Welsh immigrants, and she arrived in Australia at the age of 5. She has a remarkable Australian accent which one Daily Telegraph writer described as "tortured Strine that makes her sound like the love child of Bob Hawke and Dame Edna".
She has held together a rag tag minority Government since 2010 with the help of the Greens, elected independents, and a couple of discredited MPs who have been forced to go independent.
One of the MPs she depends on, New Zealand-born Craig Thomson was charged this week with 154 counts of fraudulent use of a credit card when he was with the Health Services Union.
Last week she tearily announced the departure of two loyal senior ministers at the next election.
She has upset party loyalists in the Northern Territory by making a "captain's pick" to replace the current sitting Labor senator with former Olympian Nova Peris as the first Aboriginal woman in federal politics.
And Rudd supporters may be stoking another destabilisation campaign against the woman who took his job.
So much of Gillard's leadership has been defined by her relationship with Rudd that they are even making a TV series about it. Unfortunately for Gillard, it is being scripted by a writer, Bob Ellis, who chastised Gillard on his Table Talk blog for her "girly tears" over the death of her father, and using his death to "take time off playing hooky from her national obligations".
The last time Key and Gillard met was in November at the East Asia Summit in Phnom Penh where they (and Craig Emerson) attended a side-meeting chaired by US President Barack Obama on the Trans Pacific Partnership talks.
Key and Gillard get on very well despite being from the right and the left, or as is often the case with transtasman relations, because of it.
They are text buddies, have the obligatory wager on sports events and Key suggested she could give New Zealand a coalmine in 2011 if the Warriors beat Manly in the NRL championships.
Key gets on with everybody so their good relationship is no surprise. But Gillard is held in special affection among politicians in New Zealand, mainly because her predecessor Rudd was relatively neglectful of the relationship.
He went everywhere except New Zealand. He sprung a plan for an EU-style Asia-Pacific on his neighbours without ever having been to a regional meeting. He was seen as cocky and anxious to get into the big league.
Finally, three years into the job, in 2010, he planned an official visit to New Zealand to speak in the New Zealand Parliament but Gillard replaced him as leader and Prime Minister, and he never made it.
It took just six months for Gillard to visit New Zealand as Prime Minister and she delivered a moving speech to Parliament about the depth of the mateship between the two countries. It was all the more resonant because of each country's responses to the Pike River mining disaster and the Aussie floods and re.
"Australia has many alliances and friendships around the world ... but New Zealand alone is family,"she said.
However, the sentiment hasn't yet translated to a lifting of the restrictions that see Kiwis arriving after 2001 denied access to welfare, student loans, housing assistance and the national disability insurance scheme due to begin this year.
And with another election looming in September and fiscal pressures remaining strong, it's not likely to be a priority.
Gillard's Government can expect to be hammered all the way to the elections for its admission just before Christmas that it will not be able to return to surplus in the current financial year. She may regret not only having made the promise, but reinforcing it as a "no ifs, not buts" promise.
As well as visiting early on as Prime Minister, Gillard led a large ministerial delegation to New Zealand in 2008 as Deputy Prime Minister - making up for Rudd having sent no minister to a celebration in Auckland to mark the 25th anniversary of the CER agreement.
Former Labour president Mike Williams remembers dinner with Gillard during that trip at the Australian High Commissioner's residence. "She called me comrade and treasured me like a long-lost mate though I don't recall meeting her before."
He remembers a conversation with her about a proposal by Finance Minister Michael Cullen to index taxation.
"What she said to me was that Cullen had taken leave of his senses because what he was doing was killing the Treasurer's best friend, bracket creep [in which inflation boosts the Government's tax take]."
During that trip Gillard praised then Prime Minister Helen Clark as an inspiration. There are similarities between the two; they are controlled and focused women who made a positive decision not to have children in order to concentrate on their careers. They both rose to the top of their parties from the left wings of their parties, but are not quite the radicals or feminists they are perceived as being.
Gillard's conservative streak was evident in her opposition to same-sex marriage, support for which is gathering pace in Australia, Britain, the US and New Zealand. The compromise her party has reached is to make it a conscience vote.
As Education Minister she championed the MySchools website comparing results of schools across the country, which horrifies many left-leaning teachers.
And her adamant opposition to the UN vote on recognising Palestinian statehood - in contrast to strong support among her backbench - forced Australia to abstain on the vote last year.
Clark won three terms of Government and went down in history as one of the longest serving Prime Ministers of New Zealand.
Gillard's political luck appears to be running out. She may just go down in history.
On the agenda
* Australia's Prime Minister Julia Gillard is in Queenstown this weekend for annual talks with Prime Minister John Key.
* The meeting marks the 30th anniversary of the Closer Economic Relations agreement.
* Besides economic matters, discussions will cover defence and security issues and co-operation on cyber-security.
* Also on the agenda are issues in the Pacific region, Australia's upcoming presidency of the G20 and its seat on the UN Security Council, and New Zealand's bid for a Security Council seat.
* The pair are also likely to touch on the welfare rights of New Zealanders in Australia, and the Australian Crime Commission's report on performance-enhancing drugs in some sports codes.
