PM Jacinda Ardern and Canada's Justin Trudeau talk women's rights in London

During a workshop a group was asked to put their hands up if they were a feminist. Prime Minister Jacinda Ardern and Canadian Prime Minister Justin Trudeau both raised hands. Photo / Claire Trevett

It is the centenary of women's suffrage in London and Prime Minister Jacinda Ardern wasted no time pointing out New Zealand had a 25 year head start.

Ardern appeared alongside Canadian PM Justin Trudeau speaking to 80 secondary school students at City Hall in London - and had bragging rights over him because it had taken Canada until 1940 to allow all women to vote.

"In New Zealand we are celebrating 125 years of women's suffrage and in spite of being really proud of that we have to be a careful we are not complacent. Even having a female PM doesn't not mean we have achieved equality."

The two Prime Ministers first spent some time mingling with the students in workshop sessions on issues such as feminism before taking part in a q and a session.

During the workshop session, one group was asked to put their hands up if they were a feminist - Ardern's hand shot up as did Trudeau's - he remarked that men can be feminists too.

Ardern said as long as there was a gender pay gap and women at risk of domestic abuse there was inequality.

That had to be achieved for the sake of all children, including her own child: "If I say children, Clarke gets nervous."

The PM's partner Clarke Gayford also took part in the workshop sessions, arriving while the PM was in a meeting with Khan.

The first question to Ardern was from a girl who wanted to know what she had to do to become PM.

Prime Minister Jacinda Ardern alongside Canadian PM Justin Trudeau in Paris earlier this week. Photo / NZ Herald

Ardern asked them to put their hands up if they had a dream job and then whether that was the same thing they thought they would end up doing.

"My advice on becoming a Prime Minister is to never give up believing you can do it. The biggest barrier is believing that we can't"

The event was organised by London's Mayor Sadiq Khan as part of the #BehindEveryGreatCity campaign to mark the centenary of the first women getting the right to vote in the UK - although that was limited to the women over the age of 30 who were landowners, the wives of landowners and graduates of British Universities - about 40 per cent of all women.

Khan asked when she decided she would be PM and she replied "I didn't."

"I believed I could do it, I just never saw myself doing the role."

Another wanted to know who her role models were.

Ardern said her role model was her mother because the values and principles she learned from her parents had guided her since then. "In particular, I learned kindness and generosity from my mother."

Trudeau said he agreed with Ardern's assessment on how to become a Prime Minister.

He said it was important that sons as well were raised to be feminists and believe in equality.

"Men are unfairly given more opportunities, more power, more weight to what they say and do because we have an imbalance in our society."

He said men should be encourageed to use that power to ensure equality.

Ardern was also asked about lowering the voting age and youth turnout.

There was a problem with youth turnout in New Zealand, but children were concerned about political issues.

She said voting was not the only way to exercise influence.

Ardern got so many letters from children that she had a staff member especially dedicated to dealing with them: "bundles and bundles of letters."

She took those letters down to Question Time with her: "I take down my folder of children's correspondence down to the Debating Chamber so that when I'm done with all the shouty questions, I look at pictures and questions from children."

She said it was not only smiley faces, but "their worries in the world." "Turtles and straws up their noses, plastic bags in the sea."

"Those letters have an impact on me, I'm talking about them now. There is power in petition and in letters and there is no age barrier on those.

"You don't have to vote to have power."

In Canada women in some provinces got the right to vote in 1916 but it was not until 1940 that all women got it.

Trudeau said the anniversary was major milestone. "But it also throws into sharp relief the work we still need to do to make gender equality a reality."

In a statement, Khan said he was delighted to host the two Prime Ministers and had looked forward to hearing what they had to say about removing the barriers facing women still.