Victoria's Environment Minister has described the deaths of at least 40 koalas at a blue gum plantation in the state's south-west as "a crime", vowing to bring to account those responsible.

Key points: Premier Daniel Andrews called the deaths a "terrible outcome" and promised a thorough investigation

Premier Daniel Andrews called the deaths a "terrible outcome" and promised a thorough investigation A company that harvested blue gum on the property said the koalas were in good health when it left

A company that harvested blue gum on the property said the koalas were in good health when it left Vets from a specialist koala hospital in New South Wales have headed to Victoria to help treat injured animals

Lily D'Ambrosio said she was "angry" about the deaths, adding that she expected many more than 40 animals to die as a result of the incident.

WARNING: This story contains graphic images.

"What I'm … deadly serious about is bringing to account every single person who is responsible for this devastation," she said.

"It is a crime, it is cruel. And it should not be allowed to be gotten away with."

Officers from the Department of Environment (DOE) were at the timber plantation near Cape Bridgewater this morning.

Portland woman Helen Oakley first raised the alarm with authorities on Wednesday after hiking into the area and finding about 10 dead koalas.

Dozens more koalas were found trapped in two isolated stretches of gum trees on the property.

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In an emotional video she posted on Facebook, Ms Oakley said 140 acres of land had been bulldozed and "[they've] just killed all of their koalas".

"Australia should be ashamed of this, and we need help," she said.

'It is a crime scene, we are treating everything as evidence'

Office of the Conservation Regulator wildfire officer Ash Burns said about 80 koalas had been assessed on-site so far and 30 of them had been euthanised.

"This is very distressing for everyone. We will take as long as we need to capture and assess every animal," he said.

"It is a crime scene, we are treating everything as evidence."

Up to 100 koalas were yet to be found, he said.

About 50 koalas that have been saved are at a wildlife shelter undergoing treatment for injuries such as fractured bones, starvation and dehydration.

Mr Burns said challenging environmental conditions, including undulating terrain, piles of logs and natural debris known as slash were making it difficult for rescuers to find and assess the remaining koalas.

Premier Daniel Andrews said the koala deaths would be thoroughly investigated. ( Facebook: Helen Oakley )

Thermal imaging cameras were being used to find dead and live animals within the slash, and climbers and flagging poles were being used to retrieve animals from the few trees that were still standing.

Mr Burns said under the Wildlife Act a koala management plan must include koala spotters pre and post-harvesting, and a number of trees must be left standing for koalas to live in before the animals moved on to another habitat.

He said it appeared that when harvesting ceased about four weeks ago, remnant vegetation had been left for the koalas but had subsequently been destroyed, leaving the animals without food.

Mr Burns said the team of vets and wildlife carers would be on-site for at least a few more days.

Koalas 'in good health' when company left

South West Fibre, a Portland-based plantation hardwood processing operation, went onto the property owned by a farmer late last year to harvest blue gum.

Company spokesman Rob Hadler said all of the koalas were in good health when the company handed the property back to the owner.

"This is the last thing we like to see in any operation. And in fact we work very hard to avoid any injuries or deaths to koalas," he told ABC Radio Ballarat.

"Everyone at South West Fibre is extremely distressed by the footage."

A number of koalas were starving when their habitat was destroyed. ( Facebook: Helen Oakley )

Mr Hadler said the company was required to have spotters, who are independent contractors, on the ground to make sure that koalas were safe.

"Those trees are quarantined from being harvested," he said.

"We report both to the Department of Environment, Land, Water and Planning at the end of our harvesting what has happened to any koalas [and] how many koalas are remaining on the property.

"There were 76 koalas on that property when we finished harvesting and … they were all in good health."

'What's happened here is wrong': Premier

Interviewed on ABC Radio Melbourne this morning, Premier Daniel Andrews called the deaths a "terrible outcome" and promised a thorough investigation, noting that killing native wildlife was an offence.

"What's happened here is wrong. It shouldn't have happened," he said.

Mr Andrews said there may have been two logging operations on the property.

"One may well have been in accordance with the rules and the other one not," he said.

"There may well have been work done by a contractor and then work done beyond that, potentially by a land owner," he said.

"This is a very bad outcome but we've got to have a proper investigation."

"Let's establish the facts. And then if there's punishment to be levied against anybody, if any offences have been committed then that investigation is all about getting to the truth and then taking action on it."

NSW vets sent to Victoria to treat injured koalas

The clinical director of the Port Macquarie Koala Hospital, Cheyne Flanagan, called for "heads to roll" over the plantation deaths.

"The Victorian Government has handed over management of koalas in blue gum plantations to the blue gum industry — that just doesn't compute," Ms Flanagan said.

"They've got to change the legislation and the way all this practice is done."

The Port Macquarie Koala Hospital has sent three vets to Victoria to assist local teams attempting to save the koalas injured at the plantation.

Ms Flanagan said the koala deaths make her "just so angry".

"It's not the first time, obviously not in this magnitude, but the issues have been going on with the blue gum plantations for many years," she said.

"They do have spotters and yes, koalas can be difficult to sight at times, but there are repeated deaths and heads have got to roll over this."

The deaths were condemned by the Australian Forest Products Association, with chief executive Ross Hampton saying he was "appalled" at what appeared to be a "callous act of animal cruelty".