The civil construction company building Sydney's troubled light rail has revealed the project has been delayed again, and will not be complete until May 2020.

Key points: Sydney's light rail project has been plagued by delays and court battles

Sydney's light rail project has been plagued by delays and court battles It was supposed to be finished before the March 2019 State election, but has been delayed several times

It was supposed to be finished before the March 2019 State election, but has been delayed several times Last month, the Premier said she was confident it would be open by the end of 2019

Bede Noonan, the Managing Director of Acciona Infrastructure Australia, told a parliamentary inquiry the NSW Government was informed of the two-month delay in August.

Last month, Premier Gladys Berejiklian said she was hopeful the project — which runs from Circular Quay to Randwick — would be open by late 2019.

It was initially supposed to be finished before the State Election, in March 2019.

Mr Noonan told the committee he was "sincerely sorry" for the disruption and pain caused by the project.

"I would like to assure the committee, residents and businesses that we regret the delays, disruption, and impacts that the Sydney Light Rail project has caused," he said.

Acciona is suing the NSW Government for more than $1 billion over claims of "misleading and deceptive" conduct.

The litigation relates to changes in guidelines provided by electricity company Ausgrid.

Mr Noonan said Acciona's biggest challenge had been dealing with Ausgrid's power assets and underground pits in George Street.

"This has had the greatest impact on the lack of certainty of the works and ultimately the disruption overall and impact on residents and businesses," he said.

The project has caused major disruption in the Harbour City's CBD. ( AAP: Mick Tsikas )

Changing guidelines sparked delays

He said shortly after contracts were signed, Acciona received new Ausgrid guidelines.

"Acciona advised the Government that the new requirements set out in the Ausgrid Guidelines were completely different to what we had together developed ... and would result in a substantial change to the contract scope," Mr Noonan said.

"Acciona estimated at that time, it would result in a delay of 865 days... and an additional cost of $426 million, to the civil works only."

He described Ausgrid's new guildlines as the "root cause" of the project's delays.

"The new pit sizes pushed the excavation works into areas which had not been investigated by the Government for utilities," he said.

"We were then discovering utilities that had not been expected."

Mr Noonan said the Government had "vastly misunderstood" the risk related to gas, electricity and sewerage utilities.

He told the inquiry Acciona also faced other "major scope changes" directed by the Government around 12 months after Acciona entered into a contract.

In total there have been more than 60 unexpected changes including alterations to lane widths and lengths on Anzac Parade and changes to tram stops at the QVB and the University of NSW.

Premier a 'liar': Foley

Transport for NSW's deputy secretary for infrastructure, Stephen Troughton, told the Upper House inquiry that he has "rejected their [Acciona's] schedule".

The last formal notification to Transport Minister Andrew Constance advised the project would be complete in March, Mr Troughton said.

"I haven't advised the minister of the formal time of when we are going to complete at the moment," Mr Troughton told the inquiry.

"I'm still holding them to March 2020 … I believe through our negotiations that we will improve on that date."

Opposition Leader Luke Foley has branded the Premier a "liar" and promised to launch a judicial inquiry to investigate the light rail "conspiracy" if elected in March.

"The public has been lied to by Premier Gladys Berejiklian and Minister Constance repeatedly on timetable, on cost, on the benefits of the project, on what's underground," he said.

He said it is the worst-delivered infrastructure project in the history of the state.

"You have to say at the very least there has been criminal neglect here by the people who presided over this project."