Junior doctors ended their third strike this morning but are balloting for what would be an unprecedented fifth strike at public hospitals.
The doctors returned to work at 8am after two-day industrial action over their employment agreement.
A fourth 48-hour strike begins on 26 February and, if approved by doctors in a week's time, the fifth strike would begin on 12 March.
But a senior advocate for their union the Resident Doctors' Association, David Munro, said they remained optimistic that a breakthrough can be reached.
Mr Munro said they need to keep the pressure on district health boards.
"We're balloting because our perception is the pressure is starting to see a change across the bargaining table, and we do remain optimistic.
"We've got a day of mediated bargaining tomorrow in Wellington and we're looking for a breakthrough. We want to see this dispute settled."
District health boards must realise the doctors would not agree to some proposed changes to the multi-employer collective agreement (MECA), he said.
"We think the DHBs need to be aware that this campaign is not a flash in the pan, it's got a lot of determination behind it.
"We suspect that the DHBs thought that the RMOs [junior doctors] somehow would run out of puff but and the campaign would lose its edge. That's clearly not the case."
District health boards' spokesperson Peter Bramley said it was disappointing and frustrating that the doctors union was considering further strike action.
"That's going to have an impact on patient care and certainly have an impact to our clinical team."
Mr Bramley said DHBs were committed to good faith bargaining and to finding a solution.
An estimated 1700 doctors took part in the third strike in their row with their employers over proposed changes to sections of employment contract dealing with rosters. The industrial action took place across all district health boards except Nelson, where it was cancelled due to the bush fires, and the West Coast.