Wildlife authorities are investigating the circumstances which have led to more than 100 koalas being put at risk near Portland.
There are reports that 10 koalas were found deceased last week at a bluegum plantation at Cape Bridgewater and 25 have been since euthanized after being assessed by vets.
About half of 120 near Cape Bridgewater have now been checked by vets.
Questions are being raised about the management of koalas in the region which has extensive bluegum plantations.
South-west MP Roma Britnell said she was made aware on Saturday night of a distressing video and images of dead and injured koalas following the harvesting of a blue gum plantation near Portland.
"As soon as I became aware I made contact with wildlife carers who were at the plantation and with locals in Cape Bridgewater who have been witnessing this over the past week," she said.
"I have gathered information from them and will take it to Minister for Environment Lily D'Ambrosio this week at Parliament."
In 2017 the State Government introduced mandatory reporting for koala deaths and injuries in the south-west region which occur during the plantation timber harvest.
A number of policies and procedures were put in place for timber companies around spotting of koalas and how to manage the harvest if koalas were present.
A spokesman for Ms Britnell said it was now understood that the forestry company which harvested the bluegum plantation had followed protocols.
Some of the koalas are trapped by a deer fence and unable to escape.
Ms Britnell said that state authorities needed to ensure that procedures were followed.
"Over a year ago the Department of Environment, Land, Water and Planning flagged with me a potential issue for population management of koalas in the south-west as the bluegum harvest cycle drew to a close, and koalas were restricted to smaller areas," she said.
"The images and video from Cape Bridgewater were horrific and unacceptable."
The Department of Land Water and Planning said in a statement that the conservation regulator was investigating the matter and it was fast tracking plans to remove the remaining koalas from the site.