Toxic chemical waste has been washed into the Yarra River at Warrandyte by Parks Victoria staff, killing trees and creating a "public health risk" near a popular Melbourne swimming spot.
According to a confidential internal report on Parks Victoria operations, a toxic cocktail of chemicals and herbicides has flowed into the Yarra from a "wash-down facility" near Pound Bend, which is a popular spot for swimming, fishing and canoeing inside Warrandyte State Park.
An incident and hazard summary report, dated October 29, reveals the concrete site is used to "pressure-wash vehicles, triple rinse chemical containers and mix herbicides for use in the park".
But the area drains directly into the Yarra, and the report states that chemical waste from the depot's wash-down area has already killed a number of trees before entering the river.
Concerns were also raised about workers being exposed to the chemical waste.
The report, released to the opposition under freedom of information legislation, noted contaminated water is washed into three separate pits that are "not designed to filter or store contaminated waste".
"The waste is manually removed, exposing the operator to unknown chemicals," states the hazard report.
The issue was recorded as far back as April 29 last year, when contaminated water was found to be draining into the Yarra River.
The report noted the wash bay fails to meet legal requirements and "if the EPA [Environment Protection Authority] was informed, PV would face serious fines".
Parks Victoria decided not to alert the EPA to the findings.
EPA acting chief executive Damian Wells said the agency was first tipped off to the issue on Friday via its pollution hotline.
"We have had an officer out to inspect the site," he said. "They have taken some soil and water samples and will be undertaking our normal investigative process."
Mr Wells said Parks Victoria was obliged to meet the same requirements under the Environment Protection Act as any private business.
"Wastewater must be retained on the site and our investigation will look at whether there have been any breaches of that requirement," he said.
"We may require a clean-up of the site and changes to the practices on the site."
A Parks Victoria spokesman said the authority will work together with the EPA to fix the issue.
He said wash bay upgrades are being scheduled after staff raised a potential safety issue with run-off.
"Local staff implemented immediate control measures to minimise any OHS risks associated with the wash bay," he said.
"Staff ceased washing down tanks and an inductor truck now comes in to remove and dispose of sediment appropriately. The use of the trucks also limits the run-off."
The spokesman said the cause of damage to the trees is yet to be determined.
Opposition environment spokesman Brad Battin said the state government needs to order a full investigation into the "environmental vandalism".
"These are shocking revelations of the Yarra being poisoned in a secret government report which Daniel Andrews has tried to bury."
But a spokeswoman for Environment Minister Lisa Neville sheeted blame to the opposition, claiming the minister and her office were only made aware of the issue when contacted by the media.
"The previous government gutted Parks Victoria, they lost one in 10 of their staff and 10 per cent of their budget, which left our parks and $1.8 billion in portfolio assets at risk," she said.
"We understand that action was taken once the issue was reported. The minister has spoken to Parks Victoria about the need to notify the EPA immediately in the future."