The ability of amateur and professional stargazers to get the best view of the heavens from a central Melbourne location is under threat from Royal Botanic Gardens expansion plans, the Astronomical Society of Victoria has warned.

The 1000-strong society, which usually shies away from public debate, has spoken out against a Botanic Gardens proposal to build a new herbarium at a cost of more than $40 million as part of its 20-year master plan.

Astronomical Society of Victoria president Chris Rudge inside the Melbourne Observatory. Credit:Josh Robenstone

The Gardens' preferred site for the underground herbarium is Southern Cross Lawn, across the road from the Shrine, on Birdwood Avenue.

The lawn is part of the two-hectare Melbourne Observatory site managed by the Gardens. The site has been the location of continuous astronomical activities since 1862.