"He's my best friend and he just shouldn't be where he is. It's just not fair."
Despite being locked up, Mr Haapu said he'd still serve alongside Australians again if he had to.
"He said to me two days ago that even after all of this, he would gladly put everything back on, put his uniform on and fight for Australia over and over again," Ms Mariner said.
"He would never ever question that because he's always going to be a soldier."
Maori Party co-leader Marama Fox told media yesterday she was contacted by Mr Haapu's family.
"He committed no crime in Australia, he committed no crime in New Zealand," she said.
"He is barely a member of the Rebels motorcycle gang, he is a decorated serviceman and he is being held in a detention centre.
"The prime minister has got it wrong and needs to do more."
Labour corrections spokesman Kelvin Davis said a number of Kiwis in Australian detention centres were upset of "being tarred with the same brush" as rapists or murderers.
He said detaining the former Lance Corporal was an example of the "politics of fear".
"It's pretty damn s*** to be honest. It's basically saying you look scary, so we're going to lock you up," he said.
"I don't really know his background at all so I don't want to comment on the specifics. I don't know him so I'm not qualified to talk."
Mr Davis said a number of the men being kept at the Christmas Island detention centre - some of whom had committed minor offences - were upset at being thought of as rapists or murders.
"In the meetings I had with them I said 'I want to know, are you an axe murderer or a rapist or anything' and they said 'No, that's the thing we're all being told we're murderers and rapists'. They were upset about it too.
"I got another Facebook yesterday from the father of another one of the guys on Christmas Island ... He said my son is not a murderer or rapist, and he's just so upset that he's being tarred with the same brush."
In 2010, Mr Haapu served in Afghanistan as a gunner and was deployed to four serious incidents where New Zealand soldiers were under attack.
One of those incidents included an attack on troops that resulted in the death of Lieutenant Timothy O'Donnell.
Melbourne lawyer Michael Pena-Rees said Mr Haapu was "deeply affected" by this incident because he personally knew Mr O'Donnell and was tasked to convey the man's body from the attack area.
Mr Haapu has been involved with the Rebels motorcycle club in Perth, where he has worked in scaffolding in a mine. His long-term girlfriend's 3-year-old son sees him as a father-figure.
A law change in Australia means foreign-born nationals can have their visas revoked by the Minister of Immigration on character grounds.
Mr Haapu was reportedly arrested on November 2.
