About half the country's junior doctors are set to walk off the job for 25 hours with every aspect of the hospital system likely to be affected.

Te Whatu Ora says emergency departments will remain open and patients should turn up to their regular appointments unless they hear otherwise.

About 2500 junior doctors are due to walk off the job from 7am on Tuesday to 8am on Wednesday.

Christchurch Hospital says it might have to postpone treatment for some patients, saying it will contact them directly.

That will likely be the case in some other hospitals, but Te Whatu Ora chief of people Andrew Slater says the agency will not know the full impact until after the strike has finished.

Health services will still be available for those who need them, he says.

Senior doctors and junior doctors from the other main union will still be working, and will be providing essential "life preserving" services.

The doctors taking industrial action are members of the Resident Doctors Association.

Senior advocate Melissa Dobbyn says they work right across the country and in every speciality.

They range from those just out of medical school, to those about to become specialists.

Most are due for a pay rise of about 20 per cent in their latest contract, but Melissa says they are striking to support a smaller group in line for a pay cut.

One of the doctors, Dr Rosa Tobin Stickings says the current pay offer is not good enough and they can't accept pay cuts for any union members.

Rosa, who is training to be an emergency registrar, says with 500 resident doctors short across the country, shifts are getting "harder and harder".

She says it's not unusual to start work and be told she has to cover for another doctor.

"It's exhausting to be working in this environment and we've been working in this environment for some years ... and we're getting pretty burnt out."

Sometimes, the doctors have to apply for annual leave a year in advance and it's difficult to get leave for education puposes or to attend conferences to help with their training.

Many junior doctors have enormous leave balances and so far Te Whatu Ora has not responded to how leave pressures could be improved, she says.

A second two-day strike is planned from May 16, and the union will take a vote on further action, says Melissa.

-RNZ.