Australia's largest autism service provider is under investigation after staff built a large wooden box to lock up distressed clients at a Melbourne day centre.

Disturbing images show the box was fitted with a metal lock and kept inside a classroom at Autism Spectrum Australia's (Aspect) site at Heatherton, in the city's south-east.

The two-metre-tall box was to have had egg cartons fitted inside for soundproofing. Clients with autism had painted the exterior as decoration.

Internal Aspect documents refer to the structure as a "desensitising box" and say it was intended to be used as a calming device.

The enclosure, described as "coffin-like" by a whistleblower,  was erected last year and dismantled in April, but the company never reported its existence to authorities.

It is now part of a wide-ranging investigation by the Disability Services Commissioner into practices at Aspect's Heatherton centre.

And Victoria's Department of Human Services has separately launched a "quality of support" inquiry into Aspect after carers allegedly used excessive force to restrain a teenage client.

Whistleblower Karen Burgess said the purpose-built box at Heatherton reflected the organisation's heavy-handed approach to vulnerable clients.

"It was just abhorrent," the former Aspect site manager said. "The box epitomises the type of practices that were occurring at that site. There was a complete disregard for the clients as human beings."

Other staff have echoed concerns to Fairfax Media about the quality of care.

The revelations are the latest to raise concerns over abuse and neglect in disability care after several inquiries were launched last year in the wake of a rape scandal at Yooralla.

A Canberra principal recently lost her job after approving a cage for a 10-year-old autistic boy, and similar allegations of forced restraint have been made against two Victorian special schools.

Nationally, Aspect has more than 10,000 clients and runs eight schools that receive government funding. The Heatherton centre runs programs for 30 people a day, aged 16 to 50, most of whom cannot talk.

In a meeting attended by Aspect employees and a client's family in July, the Disability Services Commissioner heard that "everyone at the site knew about the box".

"Staff were led to believe that it was an approved practice," the meeting was told.

The box was dismantled in April within 24 hours of being brought to the attention of Aspect's Sydney head office.

Chief executive Adrian Ford, who ordered the removal, condemned the use of illegal restraint.

"The site leader was directed to immediately arrange for the safe dismantling and removal of all materials from the site," Mr Ford said.

"The structure was in no way, shape or form suitable for its purpose, nor was it consistent with any of the approaches used by this organisation."

Mr Ford said the two staff members who built the box were stood down.

Fairfax Media has confirmed the pair were given the option to resign and left Aspect the following month. Meeting minutes show they were thanked for their service.

Head office subsequently failed to notify authorities, including the Department of Human Services, about the box.

Ms Burgess first raised alarm about the box after starting work at Heatherton earlier this year. She immediately ordered the dismantling of the box, but says the staff who built it ignored her.

"They laughed and said the box was staying ... they thought it was a good strategy," she said. "This type of thing is something you would find in a Third World country."

Aspect said its internal investigation found no evidence the box was used before its removal.

Ms Burgess has brought an unfair dismissal claim against Aspect, alleging she was fired after speaking out about the box.

In sacking Ms Burgess, the company listed serious grievances with her performance, including not working within its policies, acting outside her authority and filing paperwork late.

After hiring a crisis public relations firm last week, Aspect said it was "impressed" with Ms Burgess' decision to report the box.

"We rely on the openness and transparency of staff," Mr Ford said in a statement.

The Victorian Advocacy League for Individuals with Disability, which referred a complaint about the box to the commissioner in July, has condemned Aspect.

Chief executive Kevin Stone said he was deeply concerned that clients' families were not told about the box designed to contain their loved ones.

"VALID deplores the behaviour of this organisation, both for failing to consider and respect the human rights of its clients, and for failing to support the courageous actions of its staff."

Mr Stone slammed Aspect's treatment of Ms Burgess, saying she had been put under "enormous pressure, to the point of intimidation". "It seems beyond belief, and certainly beyond coincidence, that this former senior staff member received a letter of termination on July 15, within hours of the Disability Services Commissioner advising the service of its involvement."

The commissioner would not comment on its investigation.

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