Prime Minister Jacinda Ardern is a republican but will happily throw her support behind the Queen's successor taking over as Head of the Commonwealth when Commonwealth leaders discuss the issue this week.

The 54 Commonwealth countries are due to consider this week who will take over after Queen Elizabeth's death - and whether it will stay with the monarchy or be replaced by an elected or appointed leader.

Earlier in the week, British Prime Minister Theresa May, who is hosting those leaders, would not say if she believed it should stay with the monarch, but Ardern had no such qualms.

"It strikes me that it makes absolute sense for it to be whoever in the future takes on the role of the monarch. It just seems like a natural succession."

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She said it would also be a simpler solution than the alternatives, such as electing a leader.

That stand could save her some awkward dinner conversation when she meets with the Queen's successor - Prince Charles - and Camilla the Duchess of Cornwall tonight in London.

Ardern's position on the Head of Commonwealth is the same as former Prime Minister Sir John Key, who revealed in 2016 that in the lead-up to his 2013 visit to Balmoral Castle he had offered to lobby other leaders for the Queen to support her successor.

Ardern will be in London tonight for talks with British Prime Minister Theresa May after her Berlin visit last night. She will also take part in a joint "town hall" meeting with Canada's Prime Minister Justin Trudeau and London Mayor Sadiq Khan before heading for a meeting with May.

As well as further discussions about a post-Brexit UK-New Zealand free trade agreement, May is likely to seek Ardern's support for any joint statements about Russia and its denials of involvement in the use of a nerve agent on British soil against former Russian spy Sergei Skripal and his daughter Yulia.

Ardern faced some criticism for taking longer to specifically condemn Russia over those attacks, which Russia has denied involvement in.

Laura Clarke, the British High Commissioner to New Zealand, said New Zealand remained in the top three countries for a free trade agreement with Britain, along with the US and Australia.

"It's a real priority for both of us that we're doing this FTA. The significance for us in really in setting the tone for a future trade agreement so if we can get a really high quality, high ambition trade agreement between the UK and New Zealand it sets the tone for future trade policy." The next talks would be in April when a delegation would visit from the UK.

The proper negotiations cannot begin until next March and Britain can not sign any agreements until its transition from the EU is complete in 2020.

She said Ardern had been clear the Russia free trade agreement had now been taken off the table and the EU and UK were a priority.

There are 53 Commonwealth countries of which 16 still have the Queen as head of state. Queen Elizabeth has led the Commonwealth since 1954 - and this could be her last CHOGM as she winds back from travel.

That includes New Zealand. Ardern has said she has no plans to reform New Zealand into a republic, but believes it will happen in her lifetime.