Foreign Minister Winston Peters is in hospital in Australia with a mystery illness, possibly a tropical disease.

Mr Peters is said to be at The Wesley Hospital in Brisbane -- one of the largest private hospitals in Queensland -- after he was admitted late on Saturday afternoon.

Prime Minister Helen Clark revealed this afternoon that Mr Peters' partner had told her the minister, who is also NZ First leader, believed he contracted the illness during his recent trip to Malaysia for the Asean Regional Forum.

Mr Peters was in Brisbane en route from Kuala Lumpur in Malaysia.

"Winston's view is that he was bitten by something...and he began to feel very unwell there and by the time he got off the plane in Brisbane he was extremely unwell and his partner said it was very evident that he had to be taken for medical attention," the Prime Minister said.

NZ First chief of staff Graham Harding was with Mr Peters on Saturday.

"They (the symptoms) were such that he was clearly having a degree of difficulty," Mr Harding said.

Mr Peters was to have spent Saturday evening with Queensland Premier Peter Beattie and was expected to go to the All Blacks-Wallabies rugby test in Brisbane but had to cry off.

When asked if Mr Peters had been vomiting or had a fever Mr Harding said he had not but declined to answer further questions about symptoms.

"He's a very personal, very private sort of character. He's taking advice from the medics (who are) running the tests to see what, if anything, he's contracted and then how best to treat it once it has been identified," Mr Harding said.

"Symptoms are not material at this time. The important thing for us is ensuring he gets the best possible treatment and gets as well as possible as quickly as possible."

Mr Harding, who spoke to Mr Peters by telephone last night, described him as "unwell".

Helen Clark also spoke to Mr Peters last night but she was more positive in her assessment. "I might say when I spoke to him he was quite cheerful, so he was obviously being kept comfortable but he had not been at all well."

She believed he was feeling better than he would have on Saturday.

She said Brisbane was a good place for Mr Peters to be in hospital as doctors had good knowledge of tropical medicine.

She said Mr Peters had thought he had been bitten during a lunch in Malaysia joking that no journalists were in sight.

Mr Harding said staff from the New Zealand Consultate-General in Australia who were in Brisbane at the time were keeping in touch with Mr Peters and offering support.

Mr Harding said NZ First MPs and staff were worried.

"Of course we are worried yes, he's our key man but we're hopeful once it's identified what, if anything, he's got then they can treat him and get him well as soon as possible."

Mr Peters is believed to be in isolation. Doctors are yet to diagnose the illness and have advised him not to travel.

Mr Peters is expected to remain in hospital for up to a week. In his absence the Prime Minister is handling duties relating to Lebanon and Deputy Prime Minister Michael Cullen was also assisting with foreign affairs.

The Wesley Hospital referred calls to its public relations officer who did not return calls.

Dr Jenny Visser of the Travel Doctor, a medical centre specialising in travel, said there were a variety of illnesses Mr Peters could be suffering from but believed malaria extremely unlikely.

"The risk of malaria in Malaysia is incredibly small, virtually zero."

Influenza was a possibility.

"It may or not be specifically travel-related...it might just be a flu."

Dr Visser emphasised there were many illnesses that Mr Peters could be suffering from and tests would be needed.

- NZPA