More than 500 Southern District Health Board patients will be affected by the junior doctors' strike.
The 48-hour action started today at 7am.
A statement from the board last night said more than 500 patients had been contacted over postponed outpatient clinics or elective surgery in Dunedin and Invercargill.
Urgent and acute care would be the priority in the next two days, the board said.
More than 70% of the board's 269 junior doctors are union members.
The New Zealand Resident Doctors' Association has opted to strike because of concerns over unsafe working hours.
The board said it still did not know exactly how many junior doctors would turn up to work today.
Meanwhile, DHBs are in a row with senior doctors over their pay rate for strike cover.
Association of Salaried Medical Specialists spokesman Dr John Chambers, of Dunedin, said senior doctors had been put in a difficult situation.
After talks broke down at a national level, the board started approaching doctors individually.
The union argued for a top strike cover rate of $568 per hour.
Doctors were being asked by SDHB what they thought they should get for strike cover work.
"They will pay you the $500 if you really want it, so it's caused a lot of angst among doctors.''
"SMOs would feel much more comfortable if pay rates during industrial action were negotiated at a national level.
"This now means that individual [senior medical officers] and groups of SMOs are having to debate what their hourly rate should be when they work a shift normally covered by a junior doctor.
"This is quite divisive and unsettling for both staff and employers locally.
"Dollar figures mentioned sound excessive but it is part of a complex matter of industrial dispute relations and how much an employer is prepared to pay to get through a strike but keep the business running.
"This has to be considered both in the short term and long term if industrial action is recurrent or prolonged,'' Dr Chambers said.
In the statement from the board last night, it said it was using national guidelines to deal with the senior doctors' pay issue.
