AN old reservoir has been resuscitated to boost Melbourne's water supply.

The Tarago Reservoir has been reconnected to the grid with the opening of a new 97 million dollar water treatment plant at Drouin West.

It will supply up to 15 billion litres of water a year, adding 0.7 percentage points to Melbourne's dams which are now at just 26.3 per cent capacity.

The Tarago Reservoir was disconnected 15 years ago because water from its open catchment was deemed unsuitable for human consumption and the city's storages were about 90 per cent full.