Foreign buyers are driving up house prices and the government should restrict their purchasing power, the Greens say.

Affordable housing is a big issue and the Greens say New Zealand should follow Hong Kong's lead and bring in a 15 per cent tax on sales to foreigners.

"Many of these buyers may not even be intending to live in the homes they've bought," co-leader Russel Norman said on Wednesday.

"Real estate agents in Auckland are raising concerns that overseas buyers are driving up house prices."

Dr Norman says Australia also has restrictions on overseas buyers - they can't purchase established dwellings as investment properties unless they meet strict criteria.

"The situation, as it stands in New Zealand, is that foreign buyers only need approval under certain conditions such as when the deal is worth more than $100 million," he said.

The government earlier this month announced a plan to free up more land for housing and Labour has announced a policy to build 100,000 affordable homes over 10 years.