Sydney developers, councils and residents have been in the dark for months about where stations will be built for a new $18 billion-plus metro rail line between Parramatta and central Sydney.

But mapping of geotechnical investigations over the past year for the Sydney Metro West project gives some insight into where the state's transport bureaucrats are looking to build stations and the alignment of the line along which driverless, single-deck trains will run.

A proposed metro line from Sydney's CBD to Parramatta has been slated for opening in the second half of the 2020s.

The amount of drilling at Zetland in the south east suggests the government is serious about extending the line there from the CBD, boosting transport links to the fast-growing Green Square precinct.

But some industry insiders say Zetland could be used as an entry point for tunnelling for the mostly underground line, and provisions made for a station to be built there in the longer term.