Authorities will this afternoon use an explosive charge to euthanase a 9.5-metre humpback whale stranded on Western Australia's south coast.

The whale has been stranded in Albany's Princess Royal harbour for two weeks.

The sick whale, which has been resting on a sandbar in shallow water, is deemed too large to shoot.

The Department of Environment and Conservation says sandbags will be placed over the whale's head.

A one-kilometre exclusion zone will be placed around the whale while it is euthanased.

A helicopter will be sent out to scan the harbour for sharks in case of a feeding frenzy.