The beleaguered western Sydney hospital will get an additional 10 birthing suites, 12 operating theatres and double the number of chemotherapy chairs from 15 to 30, under the crucial redevelopment set to be completed in 2021.

Work will commence immediately a new and bigger emergency department, Premier Mike Baird and Health Minister Jillian Skinner announced on Monday.

Nepean's ED is currently one of the busiest in the state and worst performing in terms of wait times for patients in need of emergency treatment. Its elective surgery wait times were also the worst in the state.

The government has faced sustained pressure from doctors and the Labor party to fund an extensive upgrade of the dated facilities at the major teaching hospital.

Clinicians warned the hospital was in crisis mode, with staff pushed to breaking point by high volumes of patients struggling to access timely clinical care.

Mr Baird said the upgrade was long overdue after "a long fought campaign".

"We will be delivering a world-class hospital right here in Nepean that will continue to serve the catchment across western Sydney," Mr Baird said.

"The population of Western Sydney is due to rise significantly in the next 20 years and our major investment will ensure we meet the healthcare needs of the region."

The redevelopment will also include a new neonatal intensive care unit and helipad, a new MRI, and a community health service.

The build will be entirely publicly funded.

Intensive care doctor and the chair of Nepean hospital's medical staff council Dr Nhi Nguyen said the announcement was a great first step towards ensuring the hospital could meet the needs of the growing population of western Sydney presenting with complex clinical needs.

"This is a result of a very strong and consistent voice from doctors, nurses and [other] staff at Nepean Hospital. We have really worked under a huge amount of pressure," Dr Nguyen said.

"We are very happy that the minister and the premier have come out to see the hospital and acknowledged that we were in desperate need of a build.

"What's going to be really fantastic for patients is they will not need to go further east [for treatment]. They'll be able to get the comprehensive services at Nepean Hospital that have been long overdue."

The Australian Medical Association NSW president Professor Brad Frankum welcomed the "substantial boost" to the hospital's ability to cope with rising pressure on its medical services.

"Western Sydney's fast-growing population and larger numbers of sicker patients have been straining the system," Professor Frankum said.

"This Government has been very good at keeping its promises on building health infrastructure."

But he said funding also needed to guarantee appropriate staffing numbers in the upgraded facilities.

"We must build hospital capacity at Nepean and throughout the state to ensure we can continue to offer the care that our population needs," Prof Frankum said.

Labor health spokesman Walt Secord said the Baird government had been "dragged kicking and screaming" by the opposition and medical staff to commit the the redevelopment and promised to monitor progress.

"We will be making sure the Baird Government sticks to its timetable and actually delivers," Mr Secord said.

Major construction on the clinical services block will begin in 2018.