The mother of a New Zealand detainee who died inside a Sydney detention centre is struggling to come to terms with his death.

Hera Peihopa has told Maori TV it's hard to believe she will soon be burying her 42-year-old son Rob, who was only recently leading a vigorous haka as his friend left the Villawood detention centre.

"I'm still coming to terms with the fact my son is deceased, until I see him it really hasn't completely sunk in," said Mr Peihopa's mother.

Mr Peihopa collapsed and died at the detention centre on Monday night.

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While Australian authorities did not identify him, they confirmed a New Zealand national died from a suspected heart attack.

But his shocked mother said she was told about her son's death by a friend and had to wait three hours before police finally confirmed the death.

"They told me he had been found just around 10pm. He had been sparring with someone and he went to sit down and collapsed and never regained consciousness. They said it was a possible heart attack, but that didn't ring true to me."

Now she was dreading telling her grandchildren their father had died inside the detention centre.

He had been recently estranged from his partner and had not seen his children this year.

"That will be hard because Rob's been estranged from his partner for about four months and he hadn't seen his boys during that time so he's been really brokenhearted," she told Maori TV.

Mr Peihopa had been in Villawood awaiting deportation to New Zealand for around nine months after serving time for an alleged police chase.

He had previously told Maori TV that he wanted to get out of the centre for his three children and live in Australia.

Labour MP Kelvin Davis said he was saddened by Mr Peihopa's death, and called the New Zealand Government to do more to protect its citizens.

New Zealand Prime Minister John Key cautioned about jumping to conclusions over the cause of the death, saying some claims did not tally with information given by officials.

NZG