The NSW government will push ahead with a $2.5 billion motorway tunnel between Kogarah and Arncliffe but doubt remains over a future extension south towards the Sutherland shire.

NSW Transport Minister Andrew Constance on Monday announced the government had signed a $2.52 billion contract for stage one, which involves tunnelling four kilometres between President Avenue in Kogarah to the WestConnex Motorway in Arncliffe.

But asked whether the government still intended to extend a second stage of the M6 motorway on to Taren Point and south to Loftus, Mr Constance said the government was still doing its homework.

“As part of doing our homework we’ll work out where we’re going to build it to,” he said.

“We’ll come back to you where we’re going to build it to. When you build a project like that you’re going to obviously cause enormous disruption and we’ve got to work through that.”

Construction on the twin tunnels will begin early next year, and be completed by 2025