Striking junior doctors say they will not back down and district health boards must withdraw planned clawbacks to their current terms and conditions.

About 3000 junior or resident doctors are on strike for 48 hours, which is affecting all DHBs apart from West Coast District Health Board, over a lack of progress in pay talks.

Strike action began at 7am.

The doctors and their district health board employers are in conflict over pay and working conditions including rosters.

The doctors' union, the Resident Doctors' Association, says the strike action is over continued attempts by DHBs to claw back employment conditions.

That includes aspects of safer hospital rosters for the doctors, agreed between the sides after strike action two years ago.

The doctors' current pay deal ceases to exist on 28 February, after which different conditions could be offered to doctors.

Resident Doctors' Association senior advocate David Munro said that was not acceptable and DHBs must return to talks and settle a fair deal.

"Doctors aren't going to back down, the strike action is going to continue and until the DHBs are prepared to withdraw those clawbacks, we are not going to see an improvement to the situation."

DHBs have struggled to implement the rosters but reject the clawback claim, saying they simply want more flexibility.

DHBs spokesperson Dr Peter Bramley said they were very hopeful there would be an agreement before the expiry date.

"We are comitted to meeting and committed to bargaining in good faith and from the DHBs' perspective convince that there's every opportunity for us to find a constructive solution together."

Many non-urgent surgeries and outpatient clinics have been deferred or postponed durng the 48-hour strike but a DHB spokesperson said the needs of acute patients would be met.

Ian Powell, executive director of senior doctors union the Association of Salaried Medical Specialists, said it was a tense stand-off that could drag on for months.

"It will go into March unless there is some de-escalation, but at the moment the differences are so great that this has become a winner-takes-all dispute and it's very difficult to find a way through those disputes unless there is a lowering of those tensions.