Nurses Organisation members had been set to vote on an agreement with the District Health Boards last month, but some were unhappy it did not include the amount of backpay they were expecting.

Now, the 40,000 members have voted to take it to the Employment Relations Authority instead.

Nurses Organisation chief executive Paul Goulter says it will also ask the authority to rule on two more problems that have emerged after talking to members - whether senior nurses are getting enough of an increase and how often gender pay will be reviewed.

Nurses feel the authority is the best place to settle the dispute and they want it done quickly, he says.

"The nurses are deeply frustrated - and I'm sure on the employers side that is the case as well."

Spokesperson for the DHBs, Tairāwhiti DHB chief executive Jim Green, says it doesn't understand what has gone wrong, because the union had agreed to wording in settlement in December.

That included lump sump payments to recognise the time taken to reach an agreement, and a process to make sure the gender pay gains are not eroded over time.

There would have to be mediation before any employment relations hearing, he says.

Nurses first started their fight for pay equity in 2018 - to address years of underpayment because they are a workforce made up predominantly of women.

A settlement appeared to have been reached late last year but problems emerged last month, when some nurses noticed they were not getting backpaid to December 2019 as they had expected.

Health Minister Andrew Little has said if the union was successful, it could add hundreds of millions of dollars to the cost of the pay equity claim.

Goulter says only about 40 per cent of nurses in New Zealand worked for DHBs and many more are waiting on the outcome of the dispute before making their own claims.

The agreement was never set in stone as the DHBs are implying, it's always a draft until the nurses voted on it, he says.

-RNZ/Rowan Quinn.