The wealth gap between New Zealand's rich and the rest has widened over the last decade, according to figures from the government statistics agency on Tuesday.

A survey by Statistics New Zealand showed the wealthiest 10 percent of New Zealanders owned about 60 percent of the total net worth for the year ended June 2015.

The same survey found the wealthiest 10 percent owned an average 55 percent of total net worth from 2003 to 2010.

The average New Zealand household was worth 289,000 NZ dollars (203,687 US dollars) in the year to June 2015, but the top 10 percent of households accounted for around half the total wealth, while the bottom 40 percent held 3 percent.

"While some people have high net worth, around 5 percent of households had negative net worth, which means they owe more than they own," labor market and households statistics senior manager Diane Ramsay said in a statement.

The most-valuable asset was the house, which made up 59 percent of all non-financial assets, and half of all households owned the house they lived in.

But housing also contributed the most to debt levels - mortgage debt was over 60 percent of household liabilities.

"Overall, for every 1 NZ dollar (71 US cents) of assets they have, New Zealand households have 12 NZ cents (8 US cents) of debt," Ramsay said.

Opposition lawmakers warned that the growing inequality could destabilize the country, and the effects were already being felt in the housing crisis in the largest city of Auckland.

"The ramifications of inequality have become clear in recent days in the Brexit vote and also in the way the US presidential primaries have played out," said finance spokesperson for the main opposition Labour Party Grant Robertson.

"One of the consequences of inequality is that it creates a large group of disenfranchised people who feel forgotten and alienated. They then become easy targets for peddlers of fear and hate," Robertson said in a statement.

"New Zealand's inequality is worse than the UK, Australia and Canada. In New Zealand the net worth of the bottom 20 percent is less than a quarter of the net worth of the top 20 percent," he said.

"And it's only going to get worse. The bottom 20 percent owe more than they own, while the top 20 percent own more than 20 times what they owe. This presents a risk to low-income households if interest rates rise."