The stranding of 150 whales at Hamelin Bay, some 315 kilometres (195 miles) south of Perth, was spotted by a commercial fisherman at daybreak, with fears the carcasses will attract sharks.
Western Australia state's Parks and Wildlife Service said its staff were on site and assessing the health and well-being of the 15 still alive.
"Most of the whales beached themselves on dry land overnight and have not survived," said incident controller Jeremy Chick who added they they were awaiting support and equipment to help in a rescue attempt.
"The strength of the animals and the windy and possibly wet weather conditions will affect when and where we attempt to move them out to sea.
"The main objectives are to ensure the safety of staff and volunteers as well as the whales' greatest chance of survival."
Hamelin beach, named after French explorer Jacques Felix Emmanuel Hamelin who sailed through the area in about 1801, was closed with a shark alert issued for the area.
The reason why mass stranding occur is still unknown.
There are many theories including the shape of the coastline being a contributing factor, whales responding to distress calls from other whales, or groups following a leader into shore.
The largest mass stranding in Australia's west was at Dunsborough in 1996 when 320 long-finned pilot whales came ashore. All but 20 survived.
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