UPDATED 6.12PM The Prime Minister is warning against people jumping to conclusions about the death of a kiwi man in an Australian detention centre.

Rob Peihopa died in the Villawood Detention Centre in Sydney overnight.

John Key said he's been told about the death and said details are sketchy.

"While I don't have a huge amount of information I just caution people about jumping to conclusions about what caused the death. Certainly some of the claims I've seen don't tally with the information I have at the moment. But the only way we can only get the bottom of it is when we get the full report from the coroner".

A New Zealand Foreign Affairs and Trade spokesman said they're in touch with the relevant Australian authorities and won't be releasing any further details today.

A friend said by the time paramedics came to the aid of Mr Peihopa, it was too late.

Friend, and fellow detainee Vaelua Lagaaia said Mr Peihopa was visiting others in a separate compound, when he yelled out to an officer.

Labour MP Kelvin Davis said he received a phone call at 3am from a person who knew the man, saying he had passed away inside the detention centre.

"I believe there may have been a bit of a fight. I'm not sure what the fight was over, but I'm told the guy who passed away is in his 40s, relatively fit.

"There were police all around the detention centre investigating."

Mr Davis said authorities need to be up-front with the man's family about how he died.

Mr Lagaaia said Rob was fit and popular - but you could see the stress of being detained on his face.

"A lot of people don't realise but we're like ducks in a pond, we're all happy and cruisy on the outside but on the inside, no one really knows what we're going through. Unfortunately, something like this was bound to happen, purely because of the stress levels in a place like this".

Mr Lagaaia said the only time Rob raised his voice was when he watched the Chiefs play rugby.

He said other detainees feel deeply sorry for his grieving family.

"We just feel sorry for his family for what they're going through because it's not a nice thing to pass away in a place like a detention centre. It just feels weird that a Kiwi actually passed away in an Australian detention centre. It feels weird big time".

Australian correspondent Murray Olds told Larry Williams many non-Australian citizens are still awaiting detention or release.

"You've got about 40 Kiwis now being held on Christmas Island and around 200 more who are detained in these immigration detention places across Australia".

Mr Olds said Australia's opposition party is also asking for more information.

"The Department of Immigration has expressed to me that it will make no further comment at this point because the matter is now the subject of a coroner's inquiry. Labour's demanding a briefing on it, the opposition".

The death has been referred to the New South Wales Coroner.