A heart attack was not the cause of a New Zealander's death in a Sydney detention centre, a Green MP said she had been told.

The Australian Immigration Department said Rob Peihopa, 42, was suspected to have died from a heart attack in Villawood Immigration Detention Centre on Monday night.

Green MP Marama Davidson had been speaking to Peihopa's sister, and said a coroner had ruled this out.

"The coroner told Rob's family that he had sustained a head injury. But he wants to further investigate," she said.

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Four people were arrested yesterday in relation to Peihopa's death.

Ms Davidson said the family had not been able to see his body, and she was urging the New Zealand Government to intervene.

"They have been blocked at every juncture," she said. "They just feel like no one actually cares that this happened."

The family had been told that there was no CCTV footage of Peihopa's last moments or his movements in the preceding hours.

The Ministry of Foreign Affairs on Tuesday said that the New Zealand High Commission was in touch with Australian authorities.

Peihopa was serving 10 months' jail for dangerous driving charges. He had four children.

He was one of hundreds of New Zealand expats who have been captured by a controversial Australia law change which required the deportation of some convicts or ex-convicts, no matter how long they had lived in the country.

"For a long time now we've heard about mental illness, stress, self-harm from the detainees," Ms Davidson said.

"What will it take before someone shows they care about what is happening to New Zealanders?"

NZH