Junior doctors' rosters of up to 12 consecutive shifts and 16-hour days are unsafe, both for them and patients, the doctors' union says.

The Resident Doctors Association and district health boards are in mediation over pay talks.

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The association said junior doctors may be required to work seven consecutive 10-hour night shifts, or 12 consecutive day shifts of up to 16 hours, with just two days off before returning to work.

It says that is not safe, but DHBs will only agree to fix night shifts, moving to four consecutive 10-hour nights over two years.

Association national secretary Deborah Powell said it was a fundamental safety issue.

"Being unsafe on nights, being unsafe on days, they're both unsafe and they both have to be fixed.

So, this 'if we give you half will you go away?' - the answer is no we're not going to go away because we're not going to leave our members and their patients unsafe on days. We're simply not going to do that.

Fatigue is a problem but not to the extent the union claims, said DHB employment relations strategy group chair Julie Patterson.

"Most of the rosters are very workable and from our understanding most of the members are quite satisified with their rosters."

DHBs say they want to deal with rosters in dispute and have worked through 80 of 140.

They say they are offering an extra $60 million over three years for the work, amounting to a pay rise of about 5 percent, on top of a good annual salary plus allowances.