New Oxfam research shows the richest one percent of New Zealanders earned 28 percent of all the wealth created last year.
The research shows that the 1.4 million people who make up the poorest 30 percent of the population barely got one percent of the national growth in wealth.
The charity's executive director, Rachel le Mesurier, said the level of inequality in the past two years had remained the same.
"All around the world there has been a significant shift [of] wealth, particularly around property, and that's something that many New Zealanders will be familiar with.
"In many cases ... people have become wealthier with actually not having to work very hard for it ... This is not fair, we should be rewarding work and not wealth."
Ms le Mesurier told Morning Report the gap between the extremely rich and the rest of the population was unacceptable.
"The poorest proportion of our society really gained no extra benefit from that new wealth last year."
Ministry of Social Development research shows the income share received by the top one percent doubled from the 1980s through to the 1990s, from five percent to nine percent.
After that it was steady or falling until 2014, with no evidence of of a sustained rise over the last 20 years.
However, Ms le Mesurier said a "key difference" between the MSD figures and Oxfam's report was the latter included assets as well as income.
"One of the reasons [the gap has increased] is property prices ... but there are others."
The wealth gap was a complex problem but tax was "one of the levers" the government could use to address the gap, she said.
"We've got a tax working group. We're very keen for New Zealand to have a conversation about tax - what is fair tax, what is balanced tax?
"A good number of [global] billionaires don't necessarily work hard ... Two-thirds of them have either been to build on their inheritance, they've actually got a monopoly."
Multinationals and tax secrecy were other targets the government could work with other countries on, she said.
The report will be released later today.