Some were at over 90 per cent occupancy - 85 is considered ideal - this morning, including the Auckland City Hospital adults' wards, and the Waikato, Middlemore and North Shore hospitals.
"There is no obvious cause, apart from population growth," said Dr Gloria Johnson, chief medical officer at the Counties Manukau District Health Board, which operates Middlemore Hospital.
"There is currently a high level of acute medical and surgical presentations at Middlemore, including those experiencing shortness of breath and chest pain.
"These people should come to hospital. It is most important for people not experiencing critical conditions to first see their GP, particularly while the strike is under way. We need to keep our emergency department for people who are seriously unwell. If it is an emergency, people should ring 111."
"We will be discharging as many people as possible today, but only if they are well enough to go."
The DHBs have not produced a national tally of the expected disruption from the strike, although based on the two-day strike in October, it seems likely that around 6000 patients will have a non-urgent procedure or an outpatient appointment postponed.
DHBs in the Auckland and Northland regions have said more than 2200 patients will be affected.
Wellington's Capital and Coast DHB said it had deferred "around 570 outpatient appointments as well as elective surgeries".
Waikato DHB is rescheduling around 400 outpatient appointments and around 100 inpatient appointments.
Bay of Plenty DHB has postponed 350 planned surgeries and appointments.
The Resident Doctors' Association said that at least 3025 of its members are eligible to strike.
Members at the West Coast and Taranaki DHBs would not strike, said the union's national secretary, Deborah Powell.
"West Coast we only have four members. Training at Taranaki is so fragile the RMOs [resident medical officers] are concerned not to tip the balance."
The DHBs, which said the reason was "insufficient support for the strike action" among members at those DHBs, calculate that they employ more than 4000 RMOs nationally, including locum and casual staff.
Powell refused to name the issues remaining in the dispute today, because "a potential settlement is so finally balanced". She has previously acknowledged that the DHBs have made concessions since the October strike, but objected over several issues. One was that in recognition of agreeing to having many of their days off during the week, RMOs wanted "at least some of our time off to be around what weekends we do have free, so we can have some meaningful quality time with family and friends".
The DHBs' spokeswoman, Julie Patterson, said the union was "putting the lifestyle interests of their members ahead of the healthcare needs of the public".
The strike
• Resident Doctors' Association members will strike from 7am tomorrow to 8am on Friday at 18 district health boards
• West Coast and Taranaki DHBs are not affected
• An estimated 6000 patients have had non-urgent healthcare, such as elective surgery or an outpatient appointment, postponed
• Some DHBs say patients should attend their appointment unless told it has changed. Others say they have contacted all booked patients whether affected or not
• Check your DHB's website for details
• Hospitals are discharging as many patients as they safely can
• Patients with emergency or acute medical conditions or injuries should still go to a hospital emergency department or call 111
• Those with a non-urgent problem should go to a GP or accident and medical clinic
• If in doubt, call Health-line, 0800 611 116
