A budget-blowout at St Vincent's Hospital Sydney will force staff to find $18 million worth of cuts as external consultants are called in to audit the hospital's services.

The hospital's financial woes blindsided doctors who spoke to Fairfax Media, after a memo from the executive in September celebrated St Vincent's on budget performance in 2016-17 "in the face of extraordinary activity levels".

St Vincent's Hospital Sydney has called in external auditors after an $18 million budget blowout. Credit:Daniel Munoz

But in an email to staff earlier this week, chief executive Anthony Schembri said "over the past months our financial position throughout St Vincent's Health Network has been challenged".

"I want to acknowledge that these cost saving initiatives will involve changes being made by our hospital departments," Mr Schembri said.