House prices rose a quarter last month over the year before, with a shortage of inventory keeping prices up.

The Real Estate Institute of New Zealand house price index, which measures the changing value of property in the market, increased nearly 30 per cent year-on-year to a new high.

The national median price rose a seasonally adjusted 25 per cent to $820,000 from $620,000 in May 2020.

"Median prices haven't significantly eased yet as many had hoped would be the case, and things are certainly not getting any more affordable for first time buyers," says REINZ acting chief executive Wendy Alexander.

Five out of 16 regions reached record median prices as did 18 districts.

Auckland's median house price rose 27 per cent to a record $1.15 million from $905,000 last year.

Other regions with record prices were Taranaki with a 29 per cent increase, Tasman at 21 per cent and Canterbury at 27 per cent.

Alexander says a lack of housing supply is continuing to push up prices, with fewer than 15,000 properties available for sale across the entire country.

"This is the second lowest level of total inventory we've seen since we began collecting records, and properties are still selling relatively quickly for this time of year which is when we usually start to see things slow down a bit," she says.

The number of residential properties sold in May was the highest number of properties sold in a May month for three years.

Gisborne was the only region with in an increase in the number of house for sale.

"While some of the regions are starting to show signs of prices easing, which will be welcome news for locals who are looking to buy, it's hard to know whether this is a direct result of the 23 March announcements or just the usual winter slowdown," Wendy says, referring to changes to the government's housing policy.

-RNZ.