It was thought the fire was regarded as suspicious when police took over the investigation from specialist fire investigators, but until yesterday it had not been confirmed.
Asked if the fire was deliberately lit, a police media spokeswoman said no further information was available for release.
Peter McPhail, of Auckland, whose son Robby was injured in the fire, said he had heard it was being treated as suspicious and was "very disappointed'' to think someone may have deliberately lit it.
"It's a pretty serious outcome really at the end of the day,'' Mr McPhail said.
"If someone did light that fire, in theory, what could have happened is all of those boys in that flat could have been killed.
"The boys have searched high and low in their own minds and can't come up with any reason why [someone would light the fire].''
He hoped someone was held accountable for the blaze.
The flat was still uninhabitable and all four flatmates had found a new flat and replaced their belongings.
Robby (19) was still at home in Auckland where he was recovering from a broken ankle and burns and lacerations to his hands after jumping from his second-storey bedroom window to escape the fire.
He was "on track'' to make a full recovery.
The fire broke out about 6am on May 14.
Two other students at the flat escaped uninjured while a third was staying across the road at the time.
Campus Watch staff went door-to-door in the student quarter in the days following the blaze to ensure all flats had working smoke alarms.
It was revealed smoke alarms originally installed at 660 were found in cupboards following the fire.
The house is the former flat of Kiwi band Six60, which formed and regularly played gigs there.
