A software issue has been blamed for the breakdown of a tram on Canberra's light rail line, just three days after it opened to the public.

Transport Canberra deputy director general Duncan Edghill said the light rail vehicle was able to continue its journey once its software had been rebooted.

He said it appeared to be an isolated incident and was not aware of any other breakdowns since the service launched on Saturday. 

Passengers on a 9.30am service from Gungahlin to the city were left trapped inside the carriage, Liz Allen, who was on board, said. 

 The light rail broke down less than a week after it opened to the public. Photo: Sitthixay Ditthavong

Dr Allen said the power cut in and out four times between when she got on at Nullarbor Avenue and when she got off at the city.

"We had an announcement from the driver that the power had been lost to the overhead wires," Dr Allen said.

"He apologised and said they didn't know how long the delays would be.

"The tram would sound like it was powering up and we'd move on. [It] probably powered down and up again maybe four times."