Hawke's Bay hospitals used inadequately sterilised surgical equipment on patients in recent days.
A batch of 91 pieces of surgery theatre equipment used between February 2 and 11 did not undergo the final step of sterilisation, the hospital said.
Fifty-five patients from across Hawke's Bay now face HIV and hepatitistests after having operations during that date.
Hawke's Bay District Health Board chairman Kevin Snee said any risk of infection was "very low". It had launched an investigation.
The DHB said it had become aware of the lack of sterilisation on Monday when a theatre nurse noticed the colour coding on the packet was wrong, and notified a manager.
