Up to 160 pilot whales beached themselves at Toby's Inlet near Dunsborough, more than 250km south of Perth, on Thursday.
Parks and Wildlife Western Australia said 26 whales have died.
There are four pods of whales spread across 500 metres of beach with another two pods of about 130 animals offshore in the ocean.
Wildlife authorities are at the scene to try and save the creatures.
"Our team is assessing the conditions of the whales that have stranded on the beach," Parks and Wildlife said in a Facebook post.
"Our teams on the water are trying to keep the animals together and away from the beach."
Whale researcher Ian Wiese is at the "terrible" scene where he saw many of the dead creatures raising concerns the toll will rise.
"It's terrible. There are many dead on the beach," the Geographe Marine Research chair told ABC Perth.
"The others are just thrashing around, there will be many many deaths in the next few hours."
He said it is the biggest stranding event he has ever seen in the area.
Update on the — Shark Safety WA (@SharkSafetyWA) #pilot #whales stranding in SW WA.The mass stranding has now risen to 50 and 100 pilot whales at Toby's Inlet near #Dunsborough.Experienced staff from #DBCA and #Perth #Zoo #vets are currently being deployed to respond to this evolving situation. pic.twitter.com/5wQeKXSeHZApril 25, 2024
Another researcher, Chris Burton, is still optimistic some of the whales will be saved as volunteers stand in the ocean splashing water on their backs.
"As long as we can see them breathing and people can roll them up so that they can breathe and are not breathing water and put water over their backs, that helps a lot," he told ABC Perth from the site.
People trying to help are urged to abide by the directions of the Department of Biodiversity, Conservation and Attractions."The highest priority at mass whale stranding events is always human safety followed by animal welfare," Parks and Wildlife said.
The last mass stranding of whales in WA was at Cheynes Beach in Albany in July 2023 where at least 90 of the mammals died.
In light of the Cheynes Beach stranding and the behaviour of pilot whales, Parks and Wildlife are concerned many of the whales at the Toby's Inlet incident will have to be euthanised.
"We always hope for the best outcome," Parks and Wildlife said.
It remains unclear why whales strand themselves.
"They do it all the time, false killer whales, pilot whales, sperm whales strand for some reason we're not sure," Mr Burton said.
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