Hundreds of passengers were trapped on a tram for more than an hour after Sydney’s new $2.9 billion light rail service broke down, prompting an evacuation by police.

A mechanical failure on a tram at Town Hall at 4pm on Tuesday triggered the closure of about one-third of the problem-plagued network.

NSW Police board a tram stalled on Sydney's George Street. Credit:Chloe Groom

While nine trams were caught up in the system failure, one was left locked for about 70 minutes, with passengers given no explanation and forced to call the police for assistance.

Chloe Groom and her five-year-old daughter were among the passengers who were trapped.