Deliberate poisoning is now suspected to be the cause of "slow and painful" joeys deaths in Dawesville.
AWARE Wildlife Rescue is calling for help after at least five joeys and two adult females died within a week near Sutherland Street and Melros Beach Road.
The Mandurah Mail previously reported that testing was underway to determine the cause.
"We can't say 100 per cent that it is deliberate poisoning - it could have been accidental," AWARE chief executive Sam De Hann said.
"But we have reason to believe it is... and it is looking like there are many more coming down with it until we can find the source of the problem."
It is looking like there are many more coming down with it until we can find the source of the problem.Sam De Hann
More than a decade ago a toxic plant caused kangaroo deaths in the area but that had now been ruled out following analysis of fluid and blood samples and necropsies by Mandurah South Vets and Department of Biodiversity, Conservation and Attractions.
"Many, many, many years ago we had a similar occurrence and found that kangaroos had been eating a toxic plant but that's not what we're dealing with now," De Hann said.
"From a veterinary point of view, a ranger's point of view and our point of view, it's not a plant but someone has put something out.
"If it is found that someone is doing this, it's a big offence."
Penalties of up to $500,000 can be imposed for individuals and $2.5 million for body corporates, according to the The Biodiversity Conservation Act 2016 (WA).
"It's devastating and it's frustrating for us." De Hann said.
"Each has suffered a very slow painful death.
"We have a joey in now because its mum has died, it's just unnecessary," Sam said. "So far it's only kangaroos, what if this then affects possums, birds and other wildlife."
If anyone sees or knows of anyone acting suspiciously contact AWARE, Crimestoppers on 1800 333 000 or DBCA.
AWARE contact numbers are: Sam 0499 900 630, Ros 0407 424 048 or Wendy 0403 240 388.