They have no rostered breaks.
These rosters led to extreme exhaustion for the resident doctors, Jonathan said.
"I believe it's unsafe for the doctors and it's also unsafe for the patients. I know colleagues - not in Whanganui - whose patients have ended up in intensive care because they've been over-medicated. I know of resident doctors who have broken down while at work from emotional exhaustion. I have colleagues who use their annual leave to try and break up the 12-day shifts because they're not sustainable.
"In my own case, I've also suffered with fatigue, and nearly had a head-on accident when I was coming to the end of a 12-day stretch."
Jonathan said the long working hours and exhaustion often made it difficult to have a life outside of medicine.
"Across the country, doctors are working up 16 hours a day. We're fortunate here in Whanganui that we're not rostered to work more than 14 and a half hours a day - that is if we finish our jobs on time."
He said the Whanganui District Health Board had addressed the issue of working nights, which meant that resident doctors work a maximum of four night shifts in a row.
"Unfortunately, a lot of other DHBs haven't followed Whanganui's lead in this."
Jonathan said the resident doctors did not want to strike, but felt they had little choice.
"The issue is important enough that we feel we have to try and do something about it. Our patients deserve better."
He said people who became doctors expected to work long hours - but there was a difference between working long hours and working unsafe hours.
A resident doctor is someone who has graduated from medical school and is in their first few years of working as a doctor. Many resident doctors are training to specialise and become senior medical consultants.
Listen: Rotorua Hospital resident doctor Tom Reynolds speaks to Newstalk ZB's Rachel Smalley
Whanganui District Health chief executive Julie Patterson said the hospital will offer the full range of emergency and acute services, plus as many elective services as can safely be provided, during the strike.
She said six surgeries and 38 outpatient clinics have had to be postponed due to the strike, with orthopaedics, general surgery, paediatrics and obstetrics and gynaecology the departments most affected.
Senior doctors will provide cover during the strike; however Mrs Patterson said patients may face long delays at the hospital.
She said people should go to the emergency department only for genuine emergencies.
"Because anyone presenting with a non-urgent illness or injury may face long delays, it's better to go to your family doctor when symptoms first appear rather than becoming seriously ill and needing emergency services."