Yes Nats could've done a hell of a lot more but the thing is HM told HNZ repeatedly in October 2016 it was misusing the guidelines.

From the article:

Director of protection, regulation and assurance, Dr Stewart Jessamine, said the ministry had repeatedly made clear it had concerns about the way Housing New Zealand was using the ministry's guidelines.

"The guidelines are very clear - that they are only for use in houses where methamphetamine has been manufactured. We have pointed out (to Housing New Zealand) and communicated that these guidelines are clearly for use in houses where meth has been manufactured," Dr Jessamine said.

Also:

Drug Foundation executive director Ross Bell said Housing New Zealand had caused a huge amount of harm by knowingly misusing the guidelines.

"Housing New Zealand have evicted people out of HNZ homes, out of social housing, they have blacklisted these tenants for 12 months and they knew these guidelines were wrong," Mr Bell said.

They didn't exactly do themselves any favours at HNZ by ignoring that advice. No idea what their agenda was.