FIRST Union and community advocacy group ActionStation are campaigning to end what they describe as "bankflation" by New Zealand's four largest retail banks: ANZ, ASB, BNZ and Westpac.

They've released a briefing paper which shows the net interest margins of those banks, which is the difference between their lending and borrowing rates, have risen to a 17-year high.

It comes as New Zealanders grapple with high inflation and a cost-of-living crisis.

FIRST Union researcher and policy analyst Edward Miller spoke to Charlie Dreaver.