Labour's Jacinda Ardern says a 45-unit apartment block in downtown Auckland has been sold for $2.5 million while the land alone was valued at $7.6m.
She has official documents showing the 1948 heritage building was sold to Auckland University for student accommodation.
"HNZ practically gave away desperately needed social housing," she said.
"The fact HNZ made the decision to sell the block of flats is one thing but to sell it for such a ridiculous price is another issue entirely."
HNZ sold the property because it was run down and would have cost $24m to refurbish.
Finance Minister Bill English, who is responsible for HNZ, says the agency clearly had better use for the money.
"We have confidence in HNZ's processes," he told reporters.
HNZ moved its tenants out in August 2013 because of leaks and falling masonry.
Ms Ardern says the argument that the building wasn't worth retaining for accommodation is undermined by the fact the university intends using it for just that.
"It will be particularly disappointing for the tenants forced to move out that Auckland University is prepared to renovate the building to make it safe enough for its students."