There's growing disquiet about the reported arrests of Fijian leaders for taking part in an event criticising the nation's constitution.

LISTEN ABOVE: Pacific affairs journalist Michael Field speaks to Rachel Smalley

The Prime Minister, Frank Bainimarama, is the architect of that constitution.

The leaders of two opposition parties and a trade unionist are reported to have been taken into custody over the gathering, which happened last week.

Our Ministry of Foreign Affairs and Trade is trying to find out more about the arrests, and get clarification on what has happened to those being held.

Foreign Minister Murray McCully has asked for an explanation for the arrests.

"Anything that contains free speech and space for legitimate political debate would be a concern to the New Zealand Government - I've simply conveyed our close interest in how this plays out."

McCully said New Zealand is monitoring what's going on, and doesn't want to derail recent progress there, by acting without the full facts.

"We've seen a lot of things that have gone really well in Fiji, including the conduct of elections."

"We don't live in a perfect world, and we want to try and keep encouraging those who are moving in a good direction."

Labour Party leader Andrew Little said he only has sketchy information, but is concerned.

"I would be very worried if just because Fijian citizens were raising questions, and even posing challenges to the constitution, that that would be grounds for arrest," he said.

Pacific Affairs journalist Michael Field told Rachel Smalley the Prime Minister seems to have gone feral after a lot of people boycotted last week's new public holiday "Constitution Day".

Field said Rachel Smalley that during John Key's state visit to Fiji, he was significantly insulted by Mr Bainimarama.

"Do we play this particular game again? I think from our regional perspective, our security etc, we've got to watch very closely here and the next day or two is going to be crucial."

Trade Minister Todd McClay has pulled out of a trade visit to Fiji after Fiji's Trade Minister announced Fiji was withdrawing from the Pacific-wide trade deal he was going to negotiate.

However, a spokeswoman for McClay said the decision to pull out was not related to the New Zealand Government's concerns about the detention of Opposition MPs and a trade union official in Fiji over the weekend.

Additional reporting from Newstalk ZB Staff