Junior doctors are threatening more strikes early next year because of a deal with a rival union.
In a first for district health boards, they have agreed to different terms and conditions for two rival unions both representing junior doctors.
Thousands of junior doctors took to the streets in 2016 and 2017 to protest against long working hours and hospital rosters that they said were unsafe.
A hard-won deal led to new rosters that restricted the number of consecutive days doctors could work from 12 to 10.
But the hotly debated rosters also ate into crucial training time for doctors, sparking formation of a new union - Specialty Trainees of New Zealand, or SToNZ.
It has negotiated a deal with DHBs under which the rosters will no longer apply to its members, who may work 12 consecutive days if they wish.
SToNZ chair Heath Lash said the rosters were too rigid.
"It leads to doctors that aren't as well trained. It leads to multiple handovers which we know are not safe for patients and we're trying to avoid all that."
Dr Lash said the agreement was based on training guidelines from the College of Surgeons, and was as safe as the hours negotiated by the Resident Doctors' Association (RDA).
"We still can't work more than 72 hours in a week, we still can't work more than 144 [hours] in two. So we have the exact same limits, it's just we can work 12 days in a row rather than ten."
But RDA national secretary Deborah Powell said there would be two unions operating in hospitals with fatigued doctors in one of them.
She said it would create uncertainty in patients who would not know if their surgeon was over-tired.
"Patients may be forced during the informed consent process to actually ask, 'Well actually how many days have you worked, before you operate on me?'"
Dr Powell said it was unworkable and feared DHBs would impose the SToNZ deal on all junior doctors if the RDA failed to reach agreement in pay talks by March.
"If the district health boards pursue the position they're currently doing then I don't think there's any doubt there will be further strike action in the New Year. This is a matter of such concern to the 3500 members that we represent."
A district health boards' spokesperson, Peter Bramley, rejected the view the hours agreed with SToNZ were unsafe.
"The first priority is indeed a safe roster, a safe working environment which delivers safe patient care but there are some significant gaps in the current arrangement, particularly for our senior trainees, around their training, around the continuity of the medical team."
About 100 members of SToNZ have until Friday 23 November to decide whether to ratify the deal.