The New Zealand Nurses Organisation will soon ask its members if they want to legally challenge an historic pay equity settlement.

The union backed out of an agreement to ratify the deal last week; bringing a years-long process to improve discriminate pay for nurses to a stalemate.

It argues a legal review, sparked by a backlash from members who had expected it to include backdated pay to 2019, has found the deal unlawful.

The government stands by the settlement as a binding commitment and has categorically ruled out allocating any more money for it.

NZNO's chief executive Paul Goulter says the union is running a series of information sessions via Zoom this week to update its members on the legal advice.

He says members will have some time to think about it before a vote is held on whether to take legal action or proceed with the deal as it stands.

"We're looking at moving to a vote of our members, probably the week after next, on whether or not to proceed immediately to the Employment Relations Authority to seek a determination, which would involve a ruling on our position about the back pay, or whether or not our members would prefer to skip that and move to a ratification process."

Goulter says it will be a majority vote, meaning either option will need the support of more than 50 per cent of the union's 40,000 members to go ahead.

RNZ has approached the Public Service Association about their position as the second party to the stalled agreement but has received no response.

Goulter says some of NZNO's members believe they are being "short-changed" between $15,000 and $20,000 under the current settlement.

"The settlement of successive collective agreements has involved a promise to pay backdating. Our members believed that involved a promise to pay full backdating. The proposal that came forward to them did not include full backdating. Our members then said they found that unacceptable.

"We sought sought an alternative legal review of that and that legal review then said based on the documentation, the undertakings been made over the past four or five years, the DHBs were obliged to make a full backdating offer and that hasn't happened."

Last week, the Health Minister Andrew Little described the nurses' union backtrack on the deal as "peculiar" and said he believed the deal was binding and lawful.

"What's surprising is given the pretty much three years it took to get to a settlement. There's two, two and a half years of technical work. Then a few months of negotiation, and agreement reached in December last year. I'm just surprised that it's taken until [now] to let the DHBs and myself know that they no longer think that it's lawful. There's something peculiar about that."

-RNZ/Anneke Smith.