The shock announcement comes amid Ambulance Victoria's protracted dispute with the paramedics' union over pay and conditions, which has been running since 2012. Mr Sassella has also been under pressure to improve deteriorating response times to urgent calls for help across the state and to make the service more financially viable.

On Tuesday afternoon, chair of the Ambulance Victoria board Linda Sorrell emailed staff to "regrettably announce" his resignation.

"After dedicating over 30 years to ambulance services in Victoria, Greg has decided to devote more of his time to his family and will leave AV on 28 November 2014," the email said.

Ms Sorrell said Mr Sassella started his career in the ambulance service in 1983 and after a succession of various roles as an Intensive Care Paramedic, including helicopter operations, he became chief executive officer for the Metropolitan Ambulance Service.

"Subsequent to overseeing the merger of the Metropolitan Ambulance Service (MAS), Rural Ambulance Victoria (RAV) and the Alexandra and District Ambulance Services (ADAS), Greg was appointed to the role of CEO for Ambulance Victoria," she wrote.

"During his career Greg has overseen significant progress in the development of ambulance services in Victoria including the creation of Ambulance Victoria, consolidation of communication systems and world leading improvements in the use of emergency medical data to deliver improved patient outcomes, particularly in the area of cardiac arrest, just to name a few."

Ms Sorrell said interim arrangements for the CEO position following 29 November would be announced in the coming weeks.

"On behalf of the Board I would like to thank Greg for his outstanding contribution and wish him and his family all the best for the future," she said.

Health Minister David Davis also thanked Mr Sassella for his leadership and dedication.

Mr Davis said his lasting legacy was the improvement of cardiac arrest survival rates.

"Victoria's rate is now among the best in the world."

He said Mr Sassella's management skills helped him bring together Metropolitan Ambulance Service, Rural Ambulance Victoria and the Alexandra District Ambulance Service to form Ambulance Victoria.

"I wish Greg the very best for the future and thank him for his diligent and innovative leadership over many years".

Labor's health spokesman Gavin Jennings said he believed there was a "large back story" to Mr Sassella's resignation.

He said Ambulance Victoria contracts had not been publicly released for three years, the industrial dispute with paramedics remained unresolved, and a lack of state government funding had resulted in the service being forced to pursue "draconian forms of cost recovery".

"This government has been at war with not only paramedics but with Ambulance Victoria this entire term. It has been crystal clear that the relationship between paramedics and the government had deteriorated but it seems that it may well be that their CEO was caught in the crossfire."

Secretary of the paramedics' union Steve McGhie said he was surprised by the announcement and described the timing of Mr Sassella's departure - the day before the state election - as "strange".

"I hope Greg has made his own decision and that someone else hasn't made it for him," he said.

With Henrietta Cook