Members of the Resident Doctors' Association have held four 48-hour strikes so far this year.
Efforts to settle their employment agreement have foundered on the issue of the union being able to reject proposed rosters.
DHBs argue the change is needed so they can be flexible with staffing, while the union is adamant its right of review ensures doctor and patient safety.
Three days of mediation talks in Auckland last week had raised hopes the increasingly bitter dispute might be nearing settlement.
However, the sessions ended on Friday afternoon with each side blaming the other for the continuing impasse.
The RDA is now balloting members on further strike action, and a possible week-long stop work has been proposed.
"A week is what is in play," union senior advocate David Munro said.
"That's quite a dent in anyone's income ... but we've got a very determined membership."
The dispute has forced the Southern District Health board to postpone and reschedule several hundred operations, procedures and appointments.
While hospitals in the Southern region have coped so far, they have been busy, and a lengthy stop-work would place huge pressure on senior doctors who have been stepping in to cover the rounds of striking resident doctors.
Earlier, DHB spokesman David Meates defended the health board's position.
"DHBs are responsible and accountable for patient care in their area. How best to provide that care is a decision for clinicians and the chief executive at a local level, not the RDA," Mr Meates said.
"No other union in the health sector has the ability to control work arrangements to the same extent, and our offer is based on the same change process applying to more than 70,000 DHB employees."
DHBs were committed to safe rosters for resident doctors, but it was important local teams could decide rosters that worked for them, he said.
"We remain ready to talk and will reach out to the RDA."
Mr Munro said the union had applied for facilitation talks - a bargaining process overseen by the Employment Relations Authority.
Those talks would not prevent a strike happening, but both sides must consider in good faith any recommendations the facilitator makes to end the dispute.
