Interim chief executive Chris Fleming said the impact would be felt ``well beyond'' the three days of the strike.
On the eve of the last strike, in October, the board disclosed it had contacted more than 500 patients to postpone appointments and procedures.
The board did not know how many of its 271 junior doctors would walk off the job at 7am.
In a statement, Mr Fleming said the board had been contacting affected patients.
``We do not yet know how many appointments, treatments and surgeries will have to be postponed, but we do know that this industrial action will have an impact well beyond the three days of the strike in terms of access to services for our patients.
``Our hospitals are open, our emergency departments are open, and urgent and acute care, including cancer treatments, intensive care unit and maternity care, will proceed during the strike period,'' Mr Fleming said.
After October's 48-hour strike ended, the board said 52 patients had a procedure postponed and 725 outpatients had a hospital appointment deferred.
Another 52 patients were not booked for an appointment or procedure once the strike notice was received.
Talks between the New Zealand Resident Doctors' Association and district health boards failed to avert the latest strike, although the parties say progress has been made.
The strike, affecting 18 of the country's 20 DHBs, centres on safe working hours and control of rosters.
