National secretary of the Resident Doctors' Association Deborah Powell said the union had been calling for an end to back-to-back 15-hour shifts on Saturdays and Sundays.
"Why we still have so much resistance to resident doctors working reasonable hours, quite frankly, is beyond me," she said.
Working more than 70 hours a week was not good for doctors or patients, Powell added.
From June 16, resident doctors will start reducing time worked on Sundays, from 15 hours to eight.
Health New Zealand said it was offering all resident medical officers a pay rise of between 3 percent and 29.3 percent.
The new industrial action follows three recent strikes by union members over pay and conditions.
Resident Doctors' Association president James Anderson said in May that its more than 3000 members were working all week and then doing back-to-back shifts on the weekend.
"And that means over the course of seven days, they will clock up about 71.5 hours which is an interesting number because the legal maximum in our contract is 72 hours, so we get right up 71.5, there's a clause in their contract that says we should work towards 60 hours but we've never managed to get the employer close to that."
