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The strike, from 7am Monday and running for 24 hours, will go ahead unless ongoing mediation talks succeed.
It would affect all services provided by technologists, except ultrasound, at Dunedin Hospital, Southland Hospital, Lakes District Hospital, some community-based health services and Dunedin Urgent Doctors.
The SDHB employs 54 technologists at Dunedin Hospital, 28 at Southland, and eight at Lakes. All but one are union members.
''They are a small but essential workforce with a massive workload and facing a recruitment and retention crisis,'' Apex union national secretary Deborah Powell said.
''The actual number of employees is about 20% below what is budgeted for nationally, because they are leaving.''
Bargaining on a new collective agreement began in November, but the union had yet to see a new offer from employers, Dr Powell said.
''We are still in mediation. We may still get something from them, but we can't second-guess the process.''
SDHB chief executive Chris Fleming said the organisation had started contingency planning to minimise the impact of the proposed strike.
Some planned procedures and outpatient appointments might be postponed, he said.
''Patients will be contacted directly if their appointments or clinics are postponed,'' Mr Fleming said.
''Anyone with a life-threatening situation will have access to the X-ray or scan they need.
''Patients coming to emergency departments who do not have a life-threatening situation will be assessed and treated by medical staff but may be required to come back at a later date ... or be sent to a private radiology service for their X-ray or scan.''
Breast screening mammography provided by BreastScreen Otago Southland would continue as usual.
Meanwhile, DHB-employed psychologists have announced that from September 3 they will stop accepting new patients and not take part in group work with patients.
The action, scheduled for four weeks, follows a nationwide overtime ban imposed on July 1, which continues until September 3.
There are 13 SDHB staff affected by the dispute.
''We know the Ministry of Health has $13.8 million allocated to mental health workforce development this year. They are dragging their feet on spending it to improve psychologists' terms and conditions,'' Dr Powell said.
''We need to return to negotiations with DHBs presenting an offer to lift wages and staffing levels for psychologists in New Zealand's health services.''