The first tram has hit the tracks on Sydney's light rail system for testing, despite construction delays.

It is being tested nightly this week on the new light rail system along Alison Road in Randwick, and will eventually undergo daytime testing as the project continues.

Transport and Infrastructure Minister Andrew Constance called the testing of the tram an "incredible milestone", despite major setbacks.

Only 14 kilometres of the 25-kilometre long light rail, which is projected to finish in April 2019, have been laid out.

Mr Constance hit out at the contracted builders, Altrac, for "not working fast enough".

"At this time we're an unhappy customer of Altrac and we expect better of them," he said.

"They're not working fast enough in terms of laying the tracks. [We're] in the middle of contractual negotiations. We are seeking to firm up their program."

Mr Constance said zones five and six of the light rail — in the heart of the CBD's George Street — would be open to pedestrians before Christmas.