Firefighters conducting unauthorised underground work caused trains to grind to a halt on the opening day of a $15 billion rail tunnel.
An estimated 75,000 passengers packed into new high-capacity trains on Sunday to ride through Melbourne's Metro Tunnel and its five sparkling underground stations.
Within two hours of it opening, services were delayed by up to 25 minutes after a "fire brigade request" near Anzac Station.
A Fire Rescue Victoria spokesperson conceded late on Monday that firefighters "accessed an unauthorised area of the Metro Tunnel".
"This access triggered a security alarm which temporarily delayed the trains and resulted in the attendance of Metro Trains Security," the spokesperson said in a statement.
"There was no pre-organised official drill or site visit planned for the tunnel on Sunday," the spokesperson said, adding that an investigation into the incident was ongoing.
Earlier, the United Firefighters Union defended the actions of the firefighters, saying the on-duty emergency workers entered the tunnel to conduct an "urgent familiarisation drill".
Union secretary Peter Marshall said the firefighters inspected a fire control room, potential hazards and suppression systems, and strategised for urgent emergencies.
The incident sparked a "few glitches" on the line, including train doors not lining up properly at one station, Metro Trains chief executive Raymond O'Flaherty said.
The agency said an unauthorised area near Hawksburn Station was entered at 11.30am on Sunday and secured shortly after.
Victorian Premier Jacinta Allan slammed the firefighters' actions as "deadly and dangerous".
"Anyone who trespassed into a live operating environment is undertaking a potentially deadly set of actions," she told reporters on Monday.
She dismissed union claims firefighters lacked equipment, knowledge or training to respond in the Metro Tunnel, pointing to the National Rail Safety Regulator's approval of the project.
The incident has been referred to Victoria Police for further investigation.
The 9km twin tunnels are the biggest upgrade to Melbourne's rail system since the City Loop opened four decades ago.
Metro Tunnel trains run every 20 minutes across five hours on weekdays and nine hours at weekends for the next two months before a "turn-up-and-go timetable" starts on February 1.
