The government admits it uses incomplete data to address the problem of housing New Zealand's most vulnerable people.
As well as a budget blow-out on emergency housing, the government has revealed it was unable to keep records of how long people waited for social housing.
The government has already used four times' its annual budget for emergency housing in three months.
Prime Minister Bill English said the scale of the problem was largely unknown when the budget was first set.
The information was difficult to get, he said.
"Because in the past governments haven't addressed these most complex social problems in a direct way, there's not much data, and, secondly, it's hard to have data about people you can't find.
"And that's increasingly the case as we do dig into these hard problems."
When asked if, given the incomplete data, Mr English expected more budget blow-outs, he said: "Well, you can expect probably more spending as we uncover significant and complex demand."
In written responses to Green Party questions, the government revealed it was not possible to track how long people waited for social housing as people's circumstances changed.
For example, if an applicant failed to supply information, or could not be contacted, their application was suspended, it said.
Community Housing Aotearoa chief executive Scott Figenshow said there was a lack of information about the demand for emergency housing, social housing and affordable housing.
"We're learning about what that number is [of people not in housing], by the high numbers of people coming forward seeking the grant assistance.
"So, in some way, it's no surprise that these figures are at higher substantial rates than what is budgeted for."
The government said it was gathering more information and would soon publish a detailed evaluation of long-term social housing.
Opposition parties questioned why it took this long.
Green Party MP Marama Davidson blamed the government for not having the information.
"That's because they are disconnected from what is happening and they haven't made any effort to find out.
"After how many agencies, communities, political parties, telling the National government that there is a housing crisis ... the National government can't even put those two words next to each other in their vocab."
The government pointed to the $300 million it has budgeted over four years for emergency housing. It said it was addressing housing problems in ways never done before.