It is Shearer's two decades as a humanitarian worker on the frontlines - first for the Save the Children Fund, then for the United Nations - that have come to the fore in the weeks leading to his win this week in Labour's leadership vote.
He has spent time dodging bullets and bombs to deliver aid in Somalia, Sri Lanka, Rwanda, Jerusalem, Lebanon and Iraq, and supporters are gleefully reciting the line that Shearer spent his professional life saving 50 million lives while Prime Minister John Key spent his making $50 million.
Shearer has tried to draw parallels between the role of a politician and a humanitarian worker. Both environments require team-building, action under pressure, diplomacy and finding broad consensus.
Questions may hover about his political inexperience, the absence of any notable presence in the debating chamber, shaky performances in television interviews.
But those who have spent time with Shearer in war zones say it is laughable to suggest he doesn't have what it takes to be Prime Minister - even though his success in Iraq was difficult to measure because the situation was so dire.
Several aid workers spoken to by the Weekend Herald paint a picture of a tireless, fearless worker who put a premium on listening to people of all stripes, and who shunned desk work for field work.
There are glimpses of this in Mogadishu Madness, made 11 months into the year that Shearer and Meyer spent delivering aid to tens of thousands of children in Somalia - efforts which saw them named as the Herald's people of the year for 1992.
Footage shows Shearer, often dressed in khakis and colourful shirts, meeting planes at the airport to unload food, or moving through crowds of malnourished women and children in the make-shift feeding camps. "Without the aid, people would die of starvation here," a young-looking Shearer - with a full head of hair - tells the camera.
"We bring it in, so we're natural targets. People have got guns, people are hungry, people have nothing."
The hands-on style is one that Shearer replicated in Jerusalem when he was head of the UN Humanitarian Office from 2003 to 2007.
UN worker Hamada Al-Bayari, who has been trapped in Gaza behind political blockades for years, said Shearer was a constant presence despite the constant threat of bombs and gunfire. "He was always going to every locality, assisting those in the camps, supporting access to basic services such as water, health and education," says Al-Bayari, whom Shearer praised in his maiden speech in Parliament as a courageous reporter to the UN. "He was always interested to see the kids in schools and in sharing with people their sense of humanity."
Stuart Shepherd, who worked under Shearer in the Middle East, said he always showed humility and never talked down to people.
"He was equally at ease with a senior member of the Palestinian Authority as with a local fishermen highlighting the inability to leave Gaza's shores.
"There were regular Israeli incursions, artillery shelling and air-to-ground missile strikes. Many among the international community in Jerusalem limited their visits to Gaza on the grounds of the insecurity, but David would try to visit the Gaza team every two to three weeks to provide guidance and characteristic good humour."
Shearer arrived as the number of checkpoints - called "closures" - were increasing in occupied Palestine. His then media manager, Juliette Touma, said the checkpoints restricted movements and blocked the effective passage of aid.
"People couldn't get to hospitals, for example, and pregnant women had to deliver at checkpoints. People couldn't access schools, universities, go to funerals or weddings.
"David started sending staff out into the field to take photos and GPS data, and bring it back to be mapped, and he built maps through which he was able to tell the impact of the closures from a humanitarian perspective using maps, facts and figures.
"He gained a lot of respect and we managed to move this checkpoint or change the route of the wall so people could have better access to services."
Shearer's office became an information hub that was constantly visited by diplomats, NGO officials and Government ministers. He was also a focal point for media, Touma said.
"He would be speaking about Gaza and had just finished speaking to a family whose house was demolished, and he was giving something that others couldn't give because he had access to the most vulnerable."
New Zealander Ross Mountain, who worked for the United Nations for 36 years, said Shearer was brave to speak out against Israeli cluster bombs during the war with Lebanon in 2006, "even though it's sometimes difficult to do as a UN official".
Shearer was in charge of delivering emergency relief to civilians caught up in the war. He called a press conference to criticise the continued bombing by Israel in the final days as "outrageous, because by that stage the conflict had been largely resolved".
Francine Pickup, who worked alongside Shearer in Lebanon, said Shearer was determined to see emergency supplies get through - if not by road, then by boat.
"The south of Lebanon was being bombarded and it was very dangerous to deliver supplies, but he persisted. He was also a strong advocate in getting Israel to stop those cluster munitions, and making sure they were mapped so we knew where they were, to prevent people stepping on them and to get them cleared."
The work Shearer did in Jerusalem was largely responsible for his promotion to head the $2 billion aid programme in Iraq in 2007.
But the challenge in Iraq was far greater. Anarchy ruled the streets outside Baghdad's Green Zone, aid workers were targets, and movement was greatly restricted. But unlike Somalia, where aid workers were few and far between, in Baghdad Shearer was in charge of a massive operation delivered through 16 UN agencies.
"In Iraq there were no easy wins," says Pickup, who joined Shearer in Iraq. "It is difficult to say that his work was a clear success ... but he raised the bar in terms of knowledge and understanding of needs, and developing a response where capacities were lacking.
"You need to protect your staff on the one hand, and you have to make sure what you're doing has an impact and relieves suffering.
"I think he was very humbled by that experience. He was clearly very shocked. We had a number of contractors who were hit by a rocket in their accommodation, and he had to go in and review the situation with all the bodies."
Juliette Touma, who also followed Shearer to Iraq, said his strategy was similar to Jerusalem - consensus-building and decisions based on good information.
"He was behind increasing the footprint of the UN, not just working from Baghdad or Kurdistan in the north, but opening hubs in Basra and more remote places. He had this mentality that we should be all over Iraq to have maximum out-reach."
But he found the security restrictions frustrating.
"He couldn't just get into his car and drive around and see the people. We weren't really in touch with the beneficiaries due to the security restrictions."
Shearer had a close call when a rocket exploded in the UN compound, killing two people close by. But Basil Comnas, who was the UN's Deputy Resident Representative in charge of Economic Recover and Diversification, said Shearer's resolve to make a difference never wavered.
"We rode together several times in helicopter gunships over some pretty rough terrain on some bad days, we hit the dirt together in Baghdad in our Kevlar body armour and helmets when the sirens warned of incoming fire.
"Throughout, David never showed the slightest fear. I saw David as a born-leader and man of honour, a gentleman whose word was his bond, and I know how well he kept his word with me even when he had conflicting interests."
Shearer also kept his sense of humour. The Constitution According to David Shearer in Baghdad, as read out by friends in a congratulatory video to him after he won the Mt Albert byelection, included rules such as: "Agreements made after several drinks shall be considered valid and binding. It's not my fault if you can't hold your beer."
Another showed his love of playing the blues: "Guitars should be considered essential tools of diplomacy. Banjos are acceptable alternatives, but a bit sissy."
When he decided to leave Iraq to challenge for the Mt Albert byelection in 2009, he called a group of UN workers to his home in Baghdad.
"He was sad to leave his core team," Touma says.
"A lot of us had left jobs and families and come to work with him, and he was sad to leave in the middle because he was starting to build a bit of momentum in terms of the UN collecting information and being more visible.
"In a way, there were regrets there. He's a humanitarian who wants to be in the field, who wants to work with those most affected by conflict. But we also very much felt that he wanted to be with his people, to be in New Zealand, to do something for his own country."
