Junior doctors have begun strike action over a dispute about rosters, but the Health Minister says he has seen no objective evidence that their working hours are unsafe.

Junior doctors have walked off the job at public hospitals across the country today, in a dispute with district health boards over rosters they say are unsafe and want changed.

Up to 2500 junior doctors, also known as resident medical officers or RMOs, started the industrial action at 7am. The strike will continue until 7am on Thursday.

The strike has meant the rescheduling of thousands of outpatient clinics and non-urgent surgeries, and hospitals have sought to reduce their likely workload during the 48-hour strike so they can focus on acute care.

Minister of Health Jonathan Coleman said he had urged both sides in the dispute to negotiate a settlement.

"I'm encouraging both parties to get back to the table and get this thing settled."

Dr Coleman said it was unacceptable if junior doctors were working in unsafe conditions, but he had seen no objective evidence that their health and safety was being compromised.

He said junior doctors worked an average of 53 hours a week, and the changes being sought would cost $60 million and require the recruitment of another 160 junior doctors.

The New Zealand Resident Doctors Association wants the maximum number of days worked in a row cut from 12 to 10, and the number of night shifts reduced from seven to four.

Dr Coleman said many of the doctors needed to cover the extra days would be part-time, and two days a week may not be enough to make the work viable, or give them sufficient hours for their training needs.

He said while New Zealand had a great healthcare system there was always more that could be done.

Hospitals will not know until later today just how many of the junior doctors who are members of the union, the Resident Doctors Association (RDA), will strike, and how many may opt to work.

Senior doctors will provide cover, although relations between their union, the Association of Salaried Medical Specialists (ASMS) and DHBs have been severely strained since a separate row late last week over the hourly rate they would be paid for the cover.

Nurses will be working as normal during the strike, and DHBs expected all nursing clinics would continue to operate unless a nurse was needed to step in and cover critical hospital services.

DHBs said a number of outpatient clinics would also go ahead.

The situation and arrangements will vary between DHBs, and their national contingency planner for industrial action, Anne Aitcheson, is urging anyone who is unsure about an appointment for an outpatient clinic or non-urgent surgery, to consult their DHB's website and call the 0800 number provided.

Anne Aitcheson said anyone who was unwell should go to the hospital as usual.

"We don't want sick people sitting at home waiting for the end of [this] period of industrial action. Your hospital is ready for you and we do want you to go.

"Conversely, if you've got a minor illness, or a minor injury, that should be managed with your GP or at your primary care setting," she said.

Life Preserving Services agreements have been signed between the RDA and all 20 DHBs. This provides up to 300 junior doctors, who are either union members or who have joined the union since the strike notice was issued, to work.

It means some junior doctors, known as registrars, will be working in intensive care units, paediatrics, obstetrics and gynaecology, surgery, and in the event of cardiac arrest, at some DHBs.

Anne Aitcheson said the agreements were used at least once before mid afternoon.

"But ... the agreements were made and the calls were made and the staff came [in]."

Hospitals have also tried to empty beds if it was deemed suitable for patients, to reduce occupancy and help hospitals reduce their workload.

RNZ also understands some of the non-urgent surgery scheduled for today and tomorrow was done last weekend at some DHBs.

Striking doctors protest in Christchurch

Junior doctors have gathered in Christchurch's Hagley Park to draw attention to their plight.

The group has a number of activities planned for the day, including a blood drive and attending preschools to provide lessons about CPR and strokes.

Jonathan Davis said junior doctors wanted their hours reduced, and there were more than enough doctors available to cope with that change.

He said senior doctors in Christchurch had been supportive of the strike action and understood the junior doctors had safety concerns.

Dr Davis said junior doctors were not doing the same job senior doctors had done ten years ago.

"Yes, the consultants of yesteryear worked very long hours but they had patients that weren't as sick, they had patients that were staying in hospital having less invasive procedures done."

At the Southern DHB, chief medical officer Nigel Millar said Dunedin Hospital was three-quarters full and calm.

He said Southland Hospital was busier, but senior doctors were working hard there too, unaffected by a separate row over their pay rates during the strike.

"They've got good support from the senior doctors, who have been unstinting and they haven't held back at all, and have been very willing to fill the gap, which is great."