London's Mayor Sadiq Khan says he is concerned about the message Britain's Government is sending to New Zealanders as the country works to negotiate its terms of divorce from the European Union.

He made the comments following a town hall-style meeting in which he invited Prime Minister Jacinda Ardern and Canadian Prime Minister Justin Trudeau to answer the questions of about 80 young Londoners on issues of gender equality.

In response to questions over the potential tightening of visa restrictions, Khan said he was concerned the wrong message was being conveyed.

"What we don't want is a toxic and hostile environment being created so that people in New Zealand get the mistaken impression that we're somehow going to become inward and insular here."

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But he also said it bothered him the UK Government had made it harder for New Zealanders to travel there.

"Of course it does and I worry about any unintended consequences or intended consequences of a policy that would make it more difficult for talent to come to London," said Khan.

"Our future is in continuing to be a diverse city, and is continuing to be a worldly place for the talent of New Zealand."

In the leadup to Brexit, the UK Government has had to make a number of changes to tighten immigration, as population growth threatened to outstrip infrastructure.

New Zealanders staying in the UK for more than six months now have to pay a surcharge to access the National Health Service (NHS) of £150 (NZ$326) for New Zealanders on the Youth Mobility Scheme or student visas, and £200 (NZ$435) for New Zealanders on other visas for more than six months.

Out of more than 150,000 New Zealanders who travelled to the UK in 2015, about 8000 were on visas allowing them to stay longer than six months.

Changes to tier 2 general visas announced in 2016 meant employers must pay an annual $2000 levy to employ immigrant workers from April in a move that could make some expat Kiwi workers less attractive to UK employers.

The "experienced" workers on these visas will have to be in a job that pays a salary of at least $63,000.

In 2014, as London mayor, Boris Johnson launched a report in the British Parliament calling for a mobility zone between New Zealand and Australia and Britain.

Khan said the relationship between London and New Zealand had gone from "strength to strength".

"Your energy, your enthusiasm - you contribute to London being the greatest city in the world and that needs to carry."

Ardern said migration and freedom of movement was one area that could be addressed in a free trade deal with the United Kingdom.

"And we did briefly discuss the potential of that FTA with the UK and the ability of new Zealand to really model what an exemplary free trade agreement could look like."

"And so there have been suggestions from Prime Minister May we would be an early cab off the rank and I would be making that point when I meet the Prime Minister," said Ardern.

In a joint conference with New Zealand media, Khan also said he spent most of his meeting with Ardern "gushing about how privileged I was to meet your Prime Minister".

He said it was an honour having a "Londoner" lead New Zealand, and said any New Zealander who had spent time living in London was a Londoner.

Ardern spent two years in London, living in Brixton, where she worked in the Tony Blair administration.

"Firstly, I spent most of the meeting gushing about how privileged I was to meet your Prime Minister. Her election was celebrated here."

An interjection from Ardern assured "it didn't cost much to get that answer".

Ardern said she was equally excited to meet Khan.

"Not only the chance to meet the Mayor, but also the chance to participate in this event which he's organised. It probably speaks to some of the common values and aspirations that we have for our young people, regardless of our different roles."