New research by Oxfam has found that New Zealand's rich are continuing to get richer, while the poor are getting poorer.

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The research revealed that New Zealand's two wealthiest men, Graeme Hart and Richard Chandler, increased their collective wealth by $1.1 billion between 2017 to 2018.

In that same period, the poorest 50 percent of New Zealanders saw their collective wealth decrease by $1.3b.

Oxfam New Zealand director Rachel Le Mesurier said the widening gap perpetuated poverty, fuelled crime and robbed opportunities from those at the bottom.

Ms Le Mesurier said the free-market economy assumed trickledown economics worked when it did not.

"We're seeing that the extreme wealthy can actually resource and fund themselves to avoid paying things like fair tax, whilst 75 percent of New Zealand's economy - small business - they are actually paying tax.

"It's not necessarily proportionally fair if we're seeing multinationals and the extreme wealthy around the world not paying their fair share."

When people did not pay the right amount of tax, crucial service providers such as hospitals and schools lost out.

It was time the government stepped up and took action to reduce inequality in New Zealand, she said.

"There are a suite of tools that governments have that they can bring in place to reduce the size of this gap."

Oxfam was interested to see governments over the world begin to publicly report on the rate they taxed multinational corporations, for the sake of transparency.

Ms Le Mesurier hoped the Tax Working Group's final report - due out early this year - would take the opportunity to recommend greater taxes on the wealthy and more transparency in the tax sector.