The Labour-led Government has confirmed its National predecessor spent $50 million housing the homeless "in motels" when it had originally budgeted just $2 million.
Housing Minister Phil Twyford called the blowout a "ticking time bomb".
The $50 million spent on hiring motel rooms does not include the additional $8.5 million purchase of four motels for transitional housing.
Mr Twyford said, since becoming minister, he's learnt of a second hidden cost - 100 frontline staff for emergency housing. He said funding for them runs out mid-way through 2018.
"Funding for 100 front-line staff for emergency and transitional housing runs out at the end of this financial year," he said.
"This underfunding helped the previous Government's Budget projections look better, but it didn't reflect the real costs of the policies."
A spokesperson for Mr Twyford says 102 staff members make up about a third of staff working with homeless families for Work and Income New Zealand. Funding for the staff members ends June 30, 2018.
Under the existing Budget left by National, funding drops from $50m a year for front-line housing services to $37 million in 2018/19, so this looks like another sector the Government will need to find more funding to cover.
In Parliament, Mr Twyford said he would discuss Budget bids with Cabinet.
He said the "long-term answer is building more houses".
"This Government's solution is more state houses and more affordable homes, so there is a decent place to live for everyone."