A late hiccup for Sydney's driverless metro train line has spurred another state to hold off on an exact opening date for its long-awaited rival.
Melbourne's $15 billion Metro Tunnel will open to the public in early December after early works to build the 9km twin tunnels and five underground CBD stations began in 2017.
The Victorian government has been tight-lipped on an opening date for months, only committing to it being later in 2025.
The opening of the $21 billion Sydney Metro was delayed by 15 days in 2024 after the rail safety regulator would not give approval before an earlier targeted opening date.
Victorian Premier Jacinta Allan said her government was reluctant to set a precise opening date as it was still awaiting final regulatory sign-off for its signature mega-rail project.
"We have looked to the north and learned the lessons from Sydney Metro," she said on Tuesday.
The rail tunnel will open to a soft launch, with 240 extra services to run alongside the existing timetable for the Sunbury, Cranbourne and Pakenham lines.
Extra services in the "summer start" period will run every 20 minutes from 10am to 3pm between Westall and West Footscray on weekdays.
Weekend services will run every 20 minutes from 10am to 7pm between Westall and West Footscray, as well as limited select services to East Pakenham and Sunbury.
Regular Sunbury, Cranbourne and Pakenham services will continue to run through the City Loop until the full turn-up-and-go timetable takes effect on Sunday, February 1.
Those lines will ripped out of the loop at that point, with peak trains running through the new tunnel three-to-four minutes apart.
Frankston services will return to the loop as the entire network's timetable is overhauled.
The summer start before the "big switch" would allow the tunnel to open safer, smoother and sooner, the premier said.
The Metro Tunnel was originally costed at $10.9 billion when announced in the 2016 state budget, but several escalations have pushed the price tag for taxpayers to $13.48 billion.
The total construction bill is beyond $15 billion after factoring in additional agreed spending from project builders.
But the exact figure remains unknown, with the premier declaring it was up to contractors to report those costs.
The initial timetable comes after the final two completed stations at Town Hall and State Library were unveiled.
Drivers were stranded for hours during testing in September after electromagnetic interference led to a shutdown.
It made sense not introduce the full service from day one as high-capacity signalling, platform screen doors and entry points were bedded down, Public Transport Users Association spokesman Daniel Bowen said.
"The last thing you would want is packed trains stuck in tunnels with thousands of people disrupted if there are technical issues," he told AAP.
Mr Bowen conceded the initial 20-minute wait for services would not give the best experience for first-time users.
Opposition Leader Bard Battin argued beginning with infrequent off-peak services was a "real problem".
Sydney Metro opened to much fanfare in August 2024 and the Allan Labor government will be hoping for a similar boost before the Victorian election in November 2026.
