Lake Pedder is expected to reach its full water level and spill later today, for the first time since November 1988.

Heavy rain has rapidly increased the lake level by about 20 centimetres in the past 24 hours, pushing it from about 87 per cent to above 99 per cent full.

Hydro Tasmania will follow standard operating rules and open the Serpentine Dam outlet gate when the water level reaches full supply level in the next 12 hours.

Hydro Tasmania's Dam Safety Manager, Chris Topham, said opening the gate will create a controlled spill into Macquarie Harbour that poses no risk to safety or property.

"While this is a novel event involving an iconic lake, we have no concerns about Lake Pedder reaching capacity for the first time in more than a quarter of a century," Mr Topham said.

“We’ve been monitoring the situation in recent weeks, and are well prepared.

"Opening the gate later today will release a controlled flow into Macquarie Harbour, via the Serpentine and Gordon Rivers.

"No-one using Macquarie Harbour should notice any difference from the controlled spill. However we've advised TasPorts and all Macquarie Harbour fish farms as a courtesy,” he said.

Water is already being transferred from Lake Pedder to Lake Gordon at maximum capacity, which also supports Hydro Tasmania’s priority of protecting and rebuilding its major storages.

That means running a controlled spill via the outlet gate is the best remaining method to help control Pedder’s water level.

Ends.