Chief operating officer Paul Welford said strikes by radiographers would affect x-rays, CT and MRI only.
An estimated 70 per cent of laboratory staff and 75 per cent of psychologists are APEX members.
Welford said potentially 15 planned MRI appointments, 30 to 35 planned CT and 150 planned x-ray appointments each day were affected.
"As we received advanced notice of the action, the majority of these patients will be rebooked within two weeks if not already done so."
He said the impact of the industrial action would be that images for unplanned but non-life threatening conditions would either be delayed for 24 hours or be sent to TRG Imaging at no cost to patients.
Welford said the Northland DHB has contingency plans in place to ensure there was least disruption to patients and to ensure they and the staff were safe.
Those plans include doing more surgeries on the days preceding the strike, focusing on people who require less x-rays done on strike days, and outsourcing surgeries and lab services.
Northland DHB is encouraging people where ever possible to see their GP first unless it is an emergency.