A hole that suddenly appeared near tunnels being built for the M6 motorway in Sydney’s south has prompted safety inspectors to be called to the site, highlighting risks contractors face, two months after major work restarted on the troubled project.

The two-metre-wide hole emerged on Monday at Rockdale on the surface of the final 244-metre section of the four-kilometre underground motorway between Arncliffe and Kogarah.

The multibillion-dollar project has been racked by years-long delays and a stand-off between the government and a consortium led by CPB Contractors. The stalemate ended in June when contractors agreed to resume major work on the final section, more than two years after a 10-metre-wide sinkhole opened up.

The state’s workplace safety regulator confirmed it was advised of a “minor soil subsidence” on the M6 motorway on Monday. “SafeWork NSW inspectors attended the site and determined no workers were put at risk,” it said.

Transport for NSW said a localised washout occurred during pre-drilling for a jet-grouting trial used to stabilise ground conditions within the project site on the eastern side of West Botany Street.

“The occurrence was contained within the construction site and is a known risk of this type of geotechnical work. The contractor advised SafeWork NSW as a precaution, although the incident was not notifiable,” it said.

The agency said the 1.5-metre-deep hole was backfilled, and the “area made safe on the same day”.

A localised washout happens when layers of sand or gravel are eroded more quickly than surrounding materials such as clay during early column excavation, which uses high-pressure water.

The latest problem comes ahead of NSW Roads Minister Jenny Aitchison appearing at a budget estimates hearing on Friday, when she will face questions about the project.

The government has already shelled out $2.78 billion on the motorway, and has set aside a further $200 million this year for the project.

Transport for NSW’s most recent target date for completing the M6 is the end of 2028, which is three years later than a previously planned opening in late 2025.

Contractors from the CPB-led consortium building the motorway, which will link to the WestConnex toll road at Arncliffe, are using the engineering technique known as jet grouting to complete tunnelling near where the sinkholes opened up.

The technique is favoured in soft ground conditions such as those at Rockdale and involves using high-pressure fluid jets to erode soil and mix it with cement.

Construction was halted on the final stretch of the M4 in March 2024 after a sinkhole opened above one of the tunnels at Rockdale. Days later another emerged about 150 metres away.

Aitchison also confirmed on Wednesday that it will cost $200 million to fix a closed section of the Great Western Highway in the Blue Mountains, which will be jointly funded by the state and federal governments. Construction of a bridge over the damaged section at Victoria Pass is due to be finished by next June.