A mining company wants to take almost 2 million tonnes of potentially contaminated spoil dug up for the Metro rail tunnel and use it to fill in two giant disused mining pits in central Victoria.

The former gold mining pits in rural Nagambie, one of which is more than 900 metres long and 50 metres deep, have been closed for more than two decades and are full of salty water.

Nagambie Resources wants to fill its closed mine pits with spoil from the Metro rail tunnel.

Nagambie Resources has pitched to the Andrews government a plan to fill the pits with the mountains of spoil that will be dug up from beneath Melbourne's CBD as the $11.3 billion rail tunnel is built.

The government is assessing the proposal.