Sydney's embattled new Metro line will start taking passengers from Monday after weeks of delays, according to the state government. 

The Chatswood to Sydenham line was initially meant to open on August 4, but days before the Office of the National Rail Safety Regulator (ONRSR) announced it had failed to receive the necessary approvals. 

It was further delayed after a firefighter suffered an electric shock while completing safety drills on the line. 

The regulator gave the $21 billion project the green light on Thursday.

"[We're] very pleased that we have received the final tick of approval from the National Rail Safety Regulator and from Fire and Rescue New South Wales — passengers can have absolute confidence that this is a safe railway," Transport Minister Jo Haylen said.

Trial runs and testing of the new line have been underway for several months.

Ms Haylen said the service will take pressure off the T1 line with the trains able to carry more passengers during the peak hour than the Sydney Harbour Bridge and the harbour tunnel combined.

"Our city hasn't changed to this extent when it comes to how we move about since the opening of the Sydney Harbour Bridge," she said.

"It will transform our city for the better."

She advised passengers to be patient and plan their trips, with details of services already made public online and the Opal app.

The M1 Line will commence services at 4:38am at Tallawong Station and 4:54am at Sydenham Station seven days a week for its first four weeks of operation.

The line is expected to cut travel times for thousands of people, with patronage expected to grow over time.

Travel between Martin Place and Waterloo should take six minutes, Sydenham to Macquarie University should take 33 minutes, Central to Chatswood should take 15 minutes, and North Sydney's Victoria Cross to Barangaroo is estimated to take three minutes.

Sydney Metro had insisted there were only a "small number" of outstanding approvals delaying the opening despite concerns from the firefighters' union that it is not yet safe for passengers.

It is unclear if the Fire Brigade Employees Union (FBEU) will support the opening, after state secretary Leighton Drury told ABC Radio Sydney he was not convinced safety concerns had been addressed. 

"Obviously there are a couple of things to clean up," he said. 

An investigation is still being carried out into an outage on the North-Western line on July 13, which left five trains stuck in tunnels for number of hours.