The Drug Foundation is questioning whether Housing New Zealand actually understands the science it's using, to decide if a house has been contaminated with P.

The Foundation was given documents which show Housing New Zealand is once again cracking down on P users in state houses.

The papers also reveal it's re-evaluating the science it uses, to classify a house as being contaminated.

Social Housing Minister Paula Bennett said it wouldn't be realistic, and dealing with contamination is more important.

The Foundation's Ross Bell said it shows the agency is more about punishment, than compassion.

"It seems they're more interested in policing their tenants using flawed science around meth contamination then they are in playing their role as the biggest social housing provider in the country."

Mr Bell said contamination could come from a user just sitting on a couch, and might not warrant a full strip-down of the house.

"That's using science that's flawed and housing NZ know that it's flawed."

"And I think the government knows this now but for some odd reason is still happy to spend 22 million dollars of tax payers money on flawed science."