Fourth-year Whangarei resident Katie Griggs, 27, described such shift patterns as "punishing". Ms Griggs was on reduced hours because she's pregnant but said the 12-day stretches left doctors "fatigued and grumpy".
"I feel concerned about my ability to make safe decisions," she said.
"It's much more likely you'll make a prescribing error when your brain is tired and it's easy to miss small details. In medicine it's the small details that really make the difference."
The New Zealand Resident Doctors Association (NZRDA) was organising the nationwide strike involving more than 3000 doctors and was asking for rosters to include a maximum of 10 day shifts followed by four days off, and a maximum of four nights in a row.
Dr Griggs was the NZRDA's Northland representative and said the doctors felt they had no option but to strike after many months of failed negotiations. The resident doctors planned to rest during the strike, and some would be talking to schoolchildren about careers in medicine.
"We're worried about the impact on patients, other doctors, our nursing colleagues. But we feel committed to making things safer for patients," Dr Griggs said.
She said Northland DHB was one of the better DHBs in New Zealand in terms of its shift patterns, but there was still room for improvement.
Resident doctors range from those in their first year out of medical school to those training in a specialist field.
The strike was announced last Monday after failed negotiations between NZRDA and New Zealand's 20 district health boards over a 2016-19 collective agreement.
The DHBs' spokeswoman, Julie Patterson, said the offer extended during these negotiations would lead to New Zealand resident doctors "having some of the best hours of work in the world".
The DHBs had agreed to split the seven night shifts and had given a "commitment" to addressing the 12-days issue. This was in exchange for other concessions.
"Despite the DHBs' offer, the union has maintained its position of expecting DHBs to pay the [residents] for the days off that will result from the roster changes," Ms Patterson said.
NZRDA national secretary Deborah Powell said the DHBs' assertion her members expected paid days off from the roster changes was "a fairly significant misrepresentation of what's going on. There were claw-backs from the employers as well".
Northland DHB chief executive Nick Chamberlain said Whangarei Hospital had "robust processes" in place to manage the strike.
"We apologise in advance for the inconvenience this will cause and reassure you that our number one consideration is for the safety of our patients."