M6 could be delayed by a further year following sinkhole problems

The twin tunnels from Arncliffe to President Avenue Kogarah were originally scheduled to be completed this year.

The Berejeklian government, which launched the project in 2018, later pushed the date back to the end of 2025, and the Minns government, on coming to office, confirmed the schedule.

The further delay is due to the development of two sinkholes on the tunnel construction route at Rockdale in March this year.

For safety reasons, tunneling has not been allowed to recommence in that area, although it is still proceeding at the Arncliffe end.

The cost of the project is also expected to blow out further due to the construction problems.

When the project was announced in 2018, the expected cost was up to $2.6 billion.

This was later revised to $3.1 billion, a figure also confirmed in last year's state budget.

A spokesman for Transport for NSW said this week the cost and timeline of the project was under review.

"Transport for NSW and its delivery partner CGU are currently assessing options for redesign of the tunnel in areas affected by subsidence and a revised construction program, both of which will impact the cost of the project," he said.

"SafeWork NSW prohibition notices still stand, and tunnelling can recommence in the areas affected by subsidence only when SafeWork NSW gives approval.

"Tunnelling is continuing from the Arncliffe site, with 84 per cent of tunnelling excavation complete across the project."

The Sydney Morning Herald reported this week two unnamed sources with knowledge of the situation said it was expected to take an extra year to complete the motorway because of the sinkholes and the difficulty of tunnelling in the soft ground.

Transport for NSW will investigate any connection between the subsidence and the M6 works.