A group of 20 New Zealand SAS troops are in Tonga where the Prime Minister has suddenly been dismissed by the King but the New Zealand Government insists it is a coincidence.

Foreign Minister Gerry Brownlee told the Herald the troops were there - for a routine exercise.

"But they are currently confined to barracks and won't be moving out of the barracks until they get notice from New Zealand that it is okay for them to do it.

"You don't want New Zealand soldiers floating around the streets at the moment, do you?"

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He said the soldiers were due to go to an outer island for an exercise on Monday.

"But we will just assess whether or not it is right for them to even be there."

Asked if the presence of the New Zealand Defence Force was completely coincidental he said "absolutely."

"I had no idea and I would certainly know if there was an action planned."

However he would not have been be informed of just a standard exercise - which was what it was.

The New Zealand armed forces had a strong relationship with the Tongan armed forces and they regularly exercised together in Tonga and New Zealand.

"They are under clear instructions that we do not want them out of the streets under any circumstances until we can understand more fully what is actually happening up there."

In a statement the Defence Force said the SAS soldiers were in Tonga for an annual training exercise from August 25 to September 8, which had been planned since November.

"The soldiers remain in the barracks at Nuku'alofa. The NZDF is closely monitoring the situation and has put the exercise on hold for the next 24 hours.

"A Royal New Zealand Air Force C-130 Hercules aircraft, with 10 crew members on board, that flew the NZDF personnel today to Nuku'alofa has become unserviceable. Parts will be flown tomorrow from Auckland and the issue is expected to be resolved over the weekend."

The King of Tonga unexpectedly today dismissed the Prime Minister, Akilisi Pohiva, dissolved the Tongan Parliament and ordered a new election to be held by November 16.

Pohiva, the former leader of the Tongan democracy movement, has been Prime Minister since 2014.

The move was gazetted today after King Tupou VI met yesterday with the Nobles, the Privy Council and Parliament.

Pohiva came under strong criticism for poor financial management and after cancelling the Pacific Games which Tonga had been planning to host in 2019.

There is also speculation among Tongans in New Zealand that he may be unwell.

Brownlee said the Government had had no indication the move was coming.

"So we will just be trying to work out over the next couple of days what it is going to mean for democracy in Tonga and what the implications will be for New Zealand."

Asked if he was concerned the dismissal could create civil unrest, he said that was always something to watch out for in circumstances like this.

"I think the next 48 hours will tell the full story of what this is all about."

Nobles make up nine seats of Tonga's 26 members of Parliament and the other 17 are democratically elected.

Melino Maka, the chairman of the Tongan Advisory Council, told the Herald said the reason for dissolving Parliament was unclear.

But he believes it could be combination of reasons: complaints about the Prime Minister's inability to lead, inability to manage his ministers, and inability to take advice. He could also have health issues.

"He appears that he is not well but ... at the end of the day he is the democratically elected leader."

And while a lot of the development in Tonga had come to a halt since the cancellation of the Pacific Games, that was not a strong enough reason, Maka said.