Tens of thousands of extra commuters would benefit from an underground railway station at South Yarra, which could be built for as little as $400 million, new research has found.

The research contradicts the Andrews government's justification for not linking South Yarra station to its $10.9 billion Melbourne Metro underground.

The rail line is to run from South Kensington to South Yarra, but does not stop at that suburb's station despite passing just metres from it.

The new findings come from one of the original architects of the Melbourne Metro rail tunnel plan, who was commissioned by Stonnington Council.