Acciona, the firm constructing the line, and its lead contractor both told a parliamentary inquiry on Thursday the project won't be finished until May 2020.

That's two months later than a revised March 2020 deadline which itself was more than 12 months later than the original scheduled opening date.

The state's chief bureaucrat for the project hasn't accepted the new date, however, and wants the numbers run again.

"A lot of the assumptions just don't stand up in their program," Transport for NSW's Stephen Troughton told the inquiry.

Acciona said it blamed the latest delay on electricity distributor Ausgrid failing to move overhead wiring in Kingsford and Kensington.

The Spanish-owned firm is currently in a legal dispute with the NSW government seeking more than $1.1 billion extra due to contract and utility issues.

Acciona Investment Australia managing director Bede Noonan said expanding electricity pits often meant having to bore into areas not surveyed pre-contract followed by a lengthy process to identify and move pipes underground.

"We dig a hole, find something, stop," Mr Noonan said.

Transport Minister Andrew Constance has previously alleged the Spanish-owned firm engaged in a deliberate "go-slow".

Mr Noonan on Thursday claimed the government knew of the latest delay two months ago.

That led Opposition leader Luke Foley to question why Mr Constance told a budget estimates hearing in late August the completion date was still March 2020.

The Labor leader has called for a judicial inquiry into the whole project.

"Frankly, both the transport minister and the premier herself should be in the dock over this," he told reporters.

The light rail inquiry earlier this week heard from residents who said noise levels had often reached up to 90 decibels overnight - in breach of regulations.

Mr Noonan on Thursday admitted Acciona only conducted noise monitoring in specific areas because that was "reasonable practice".

TfNSW co-ordinator general Margaret Prendergast said there was a "strong monitoring regime" in place.

Ausgrid on Thursday told AAP its role involved certifying the relocation of overhead cables underground and "the work is being delivered in accordance with Acciona's construction schedule".

Australian Associated Press