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 hepatitis tests 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.
It would be 24 weeks before the patients could be given the all clear.
The DHB had called a press conference for 11am.
Minister for Health David Clark is also reportedly aware of the issue.
A ministry spokesman has said it was a matter for the Hawke's Bay District Health Board to respond to.
