"He was part of a team who looked after me when I was in Afghanistan [when] we went to both Kabul and then up to Bamiyan," he said.
The Prime Minister said he did not have the full details of his case because "the Australians won't give that to us".
He pointed to a suggestion by Australian Immigration Minister Peter Dutton that there could be other details about Mr Haapu, which had not yet been made public.
"There seems to also be other information that the Australians have that the minister's pointed to. Now I don't know what that is."
Regardless, he could not act on individual cases or intervene in Australia's law-making, even though the Government disliked its detention and deportation policy.
The Maori Party has highlighted Mr Haapu's detention as an example of the injustice Kiwis were facing in Australia.
Labour's Kelvin Davis said many detained Kiwis were upset about being "being tarred with the same brush" as rapists or murderers by Mr Key.
Figures released by the Justice Minister Amy Adams showed that there were no New Zealand rapists or murderers on Christmas Island, though some had been previously jailed for manslaughter and indecent dealings with a person under 16.
Mr Key said today he would not correct his statement about rapists in Parliament because it related to the broader group of 585 New Zealanders who had their passport revoked.
Transcripts from Tuesday's question time show Mr Key made the "rapist" comments while answering questions on a broad range of topics, some of which referred to Christmas Island.
Meanwhile, Justice Minister Amy Adams has confirmed that legislation will be introduced next week which will allow the Government to impose parole-like conditions on people deported from Australia. At last count, 167 people had been sent back to New Zealand under a law change in Australia which required anyone who had served a year or more in jail to have their visa revoked.
