Chris Hipkins to meet King Charles, Prime Minister Rishi Sunak during United Kingdom trip

Prime Minister Chris Hipkins has confirmed he will be meeting with King Charles III, the UK Prime Minister Rishi Sunak, and NZDF troops training Ukrainians outside London.

The visits will form part of his trip to Britain to attend the Coronation of King Charles III.

Speaking to reporters after Monday's Cabinet meeting - apparently in a slip of the tongue - Hipkins confirmed arrangements were being made for a meeting with the King and other royals.

"I'm not sharing the details of that right at the moment, but yes I will be meeting with several members of the royal family including - Well, I just have, haven't I? including the King."

He would not confirm any further specifics including whether it would be a one-on-one meeting, what he planned to say, or which other members of the family he might meet with.

He also said he planned to take the pledge of loyalty to the King - despite believing New Zealand should eventually separate from the monarchy.

"Yes, I will, because the King is our head of state, as I've taken pledges to previously to the Queen, and subsequently to the King when I became prime minister.

"In terms of our oaths and affirmations that we have in New Zealand, the oaths and affirmations do involve a pledge of loyalty to the reigning monarch, which in this case is King Charles the Third."

Questioned on his stance on the monarchy, Hipkins described himself as a republican but said in his view such a major constitutional change would require a referendum and was not a priority at this time.

"No, I can't envisage doing it, certainly not in the next term.

"The constitutional arrangements that we have working quite soundly at the moment. A decision to become a republic would involve a whole lot of other potential decisions that could be quite distracting at a time when I think New Zealanders have indicated they want us focused on much more pressing issues."

He said it was something for New Zealanders to instigate a discussion on, and there was not yet a groundswell of support for it.

Meeting with UK's PM Rishi Sunak

He said meetings were also confirmed with Prime Minister Rishi Sunak at 10 Downing Street on Friday evening NZ time, with a focus on advancing the New Zealand-UK free trade agreement.

"Virtually all our current trade will be duty-free from entry into force, including duty-free quotas for key products like meat, butter and cheese. Therefore gaining entry into force as soon as possible will be a focus of the trip," he said.

He said they would also discuss the working holiday scheme, the war in Ukraine and New Zealand's ongoing commitment to it, and the UK's role in the Indo-Pacific.

Hipkins to visit NZDF Ukraine troop training facility

The trip will take in a visit to the New Zealand Defence Force personnel in the UK who are training frontline Ukrainian troops outside London.

He said he expected to make announcements there about further support for the Ukrainian defence of their country against Russia's war.

It will follow a phone call to Ukraine's President Volodymyr Zelensky this evening.

"I intend to outline to the President in my call this evening what shape the additional support that we're going to provide will be, and then we'll certainly share that once we've once I've had the opportunity to share it with Ukraine first," Hipkins said.

University of Otago Robert Patman told RNZ New Zealand could be doing more to support Ukraine - while the funds and military training is much appreciated and going very well, Ukraine desperately needs military hardware for a counteroffensive.

"I think it's absolutely critical," he said.

"If you contrast New Zealand's contribution with that of Australia and Canada - two countries that we often identify with - ours, even taking into account we're a much smaller country than either, is still much smaller than they've contributed."

New Zealand has provided about $7.5 million contributed for arms via the UK, plus $10.6m for non-lethal military equipment via the NATO Trust Fund and TAIT Communications, and nearly $13m in humanitarian aid.

This compares to the contribution from Australia: More than $AU475m for military assistance, $65m in humanitarian aid and $32.6m for energy security plus 70,000 tonnes coal, $1m for radiation protection equipment, and in February the promise of drones.

Canada has provided even more: surface-to-air missile systems ($CA406m), eight tanks, 208 armoured vehicles ($CA92m), 39 armoured combat support vehicles ($245m), one armoured recovery vehicle, four Howitzer artillery guns, drone cameras ($100m), ammunition and small arms (over $169m), 100 anti-tank rifles, 4200 rocket launchers and 7000 hand grenades ($7m), winter clothing ($25m), satellite systems ($24m), plus $CA15m in non-lethal equipment and more than 640,000 meal packs.

Patman said New Zealand should be sending any surplus hardware available to Ukraine.

"It's very important for the liberal democracies that Ukraine succeeds in repelling and ejecting the Russian troops from their country, and I do think it's a time where we must step up a little bit," he said.

"I mean, I appreciate there are logistical problems in transferring this weaponry and it needs to get there quickly. But if we do have surplus military hardware, I think it would be a very good move to make it available to Ukraine.

He said it was a "critical moment" in the conflict.

"The conflict's been in a holding pattern since the successful Ukrainian counter offensive last year when it retook Kherson and Kharkiv. And really since then although the Russians launched their own offensive in January, that hasn't really made much progress.

"We have a big stake in the outcome. We have a big stake in Mr Putin failing in his attempt to steal territory from a neighbor and we depend as a country ... very heavily on the rule of law internationally and the rules have been broken in this case."

He said a victorious Ukraine could also lead to reform of the UN security council system - something New Zealand has long supported - and would send the message that might is not right.

"Russia is chairing the UN Security Council, and yet the country has broken the all the rules of the UN in the last year in illegally invading a neighboring country - so there's big problems at the UN Security Council."