Just how big the state housing selldown might pan out remains a mystery, even to Housing New Zealand bosses.

The government plans to reduce the agency's stock from 68,000 houses to 60,000 by 2017.

A select committee's grilled Housing New Zealand chief executive Glen Sowry over how many might be sold off within a decade.

"Well, there's different numbers being bandied around by various people but ultimately until we get clear direction from shareholding ministers and the government as to what those numbers are, we're focusing on what we know today."