Indigenous heritage consultants are demanding an urgent halt to construction of a section of Sydney's new light rail line, after one of the largest recent discoveries of indigenous artefacts in the city.

About 20,000 artefacts have already been recovered, and heritage experts working at the site say there could be more than 50,000 at the area intended to be used to stable trams in Randwick.

"There is nothing at all like this in Sydney," said Scott Franks, whose company was contracted to advise on indigenous heritage issues for the $2.1b light rail line.

One of the significant elements of the find, Mr Franks said, was that some artefacts appeared to be from the Hunter Valley.