70-year-old sandstone sewer tunnel managed carefully on Cronulla's Vue site

The tunnel, which is about 1800mm (6ft) high and 1400mm (4.5 ft) wide, remains a main trunk for the system, which was built in conjunction with the Cronulla Sewage Treatment Plant, which began operating in 1952.

Project director Matt Crews said the tunnel, which was built in the 1940s, was "like a sandstone tube that carries the sewer for a large part of Cronulla".

"The walls and roof are rock, and so is the base but more recently it was levelled out with concrete to improve the flow," he said.

"Because it doesn't have a pipe, we had to go through an extensive process with Sydney Water to make sure that we weren't building too close to it."

Mr Crews said they knew of the existence of the tunnel through the due diligence that was carried out when they acquired the site.

"We knew there would be some careful engineering work we would have to undertake," he said.

"We built a pile shoring wall to the east of the tunnel, where the excavation is deeper, to protect it.

"It has proved no more challenging than what you do when you are excavating on a boundary, but we had to install an expensive monitoring system within the tunnel to ensure we weren't causing too much vibration.

"It has probably added about $800,000 to the cost of construction."

The tunnel is accessed from manholes in Croydon Street outside the development site.

Mr Crews said there was a similar sewage tunnel beneath the Sammut Group's Parc development on the edge of Monro Park.

"I've been working in this industry for 35 years and these are the only tunnels of this kind I have had to build over," he said.

The $380 million Vue development, opposite the northern end of Cronulla mall, will have two eight-storey towers above a podium containing 112 apartments, as well as commercial offices and a ground level shopping centre anchored by Harris Farm Markets.

Mr Crews said the project was progressing well.

"We expect to complete construction by the end of 2025 or early 2026, depending on the weather," he said.

"We have sold all eight commercial offices, and have only got one more shop to lease. Harris Farm Markets are going in there and we have got some other great shops adjacent to them.

"We are about 50 per cent of the way through residential sales."