Heavy rain pushes Tas lake into overflow

For the first time in almost three decades Tasmania's Lake Pedder is set to fill and spill.

Heavy rain has sent the lake's water levels up by 20cm in the last 24 hours and with capacity now at 99 per cent an overflow is expected late on Wednesday, Hydro Tasmania said in a statement.

"Opening the gate will release a controlled flow into Macquarie Harbour, via the Serpentine and Gordon Rivers," spokesman Chris Topham, adding that November 1988 was the last time the lake spilled.