The scheduled completion date for the 11 kilometre-long New M5 has been extended until 2020, from 2019.

The delay means that the duplication of the existing M5 East between Beverly Hills and St Peters in the inner west will not be opened until after the next state election in 2019, depriving the state Coalition of something to showcase to voters.

Sydney Motorway Corporation, the body in charge of delivering the toll road project, said the completion date had been extended "as a consequence of the environmental approval being delayed for several weeks under the caretaker provisions of the federal election".

The federal Coalition accused Labor during the election campaign of costing taxpayers $750,000 a day for refusing to give its agreement while the government was in caretaker mode for construction to start on the New M5.

It layed much of the blame at the time on Labor frontbencher Anthony Albanese, who faced a tough contest with the Greens for the inner-city seat of Grayndler where WestConnex is a significant issue.

However, Mr Albanese said on Tuesday that he understood that the federal Environment Department was prepared to approve the New M5 "but Minister [Greg] Hunt chose to politicise the decision making process" during the election.

"It is farcical to suggest that because the caretaker period was in place for a period of weeks, that that has delayed the project by a year," he said.

"What is clear is that WestConnex will be a template for how not to plan a major infrastructure project with changes, delays and cost blow outs happening on a regular basis."

Mr Albanese, who is Labor's transport and infrastructure spokesman, came under fire during the election campaign for telling inner-city residents that Labor would not commit further funds to WestConnex.

A spokesman for Mr Hunt, who recently switched from Environment Minister to Industry, Innovation and Science, said Labor deliberately stalled a decision on the project.

"The Coalition tried to consult with Labor during the caretaker period to ensure an environmental approval decision for this significant infrastructure project could be made promptly, but [shadow environment minister] Mark Butler deliberately stalled the process," he said.

Mr Hunt gave environmental approval for the second stage on July 11.

NSW Roads Minister Duncan Gay said on Tuesday that it had been well publicised that the delay in approving the New M5 during the election was stalling the start of construction.

"We'll still be working hard to try and open the New M5 in late 2019, but due to circumstances beyond our control, it now may be slightly later," he said.

"This will not impact any other stage of the project."

While it may be a setback to the Baird government showcasing the second stage of WestConnex during the 2019 election campaign, it may eventually prove to be positive from a political perspective because it will delay the introduction of tolls on the new duplicated roadway.

The WestConnex project will result in motorists having to pay directly for driving on what at present are the toll-free M5 East and M4 motorways.

A widened stretch of the M4 is due to be opened next year, while the M4 East – both of which are part of stage one of WestConnex – is slated to be completed in 2019.

The National Audit Office is also investigating whether the Coalition government received appropriate advice and value for money in committing funding to what is Australia's largest toll road project.

The federal and state governments have committed $5.3 billion in direct funding and concessional loans to the project, which has been highly controversial in Sydney's inner west because of the major upheaval to suburbs.

The cost of the tollroads, to be built over the next eight years, has climbed rapidly since a $10 billion project was proposed by Infrastructure NSW in 2012. Last year the cost ballooned from $15.4 billion to $16.8 billion.