They unchain steel gates and switch off alarms. They switch on the lights then slip on their gloves. This is life for a legion of low-paid workers who keep Victoria's government schools clean.

"Emptying bins, bleaching toilets, dusting, vacuuming, scrubbing, removing graffiti," says Victor Barrientos, 63.

"All the classrooms, all staff rooms, all the bathrooms."

Cleaners like Barrientos are often underappreciated and, it seems, routinely underpaid. Fairfax Media has interviewed cleaning staff from several large state-run schools, most of whom take home just $600 or $700 for five days' work, but are habitually expected to put in much longer hours in unpaid overtime.

Many of them are at the mercy of a shady network of contractors and subcontractors, according to workers and senior cleaning industry executives.

The cleaners reported having to work for free on a daily or weekly basis. To get the job done, they stay late into the evenings or have to come back on weekends, meaning their overall hourly pay invariably drops far below the Cleaning Services Award's legal minimum of $18 an hour.

The fresh revelations have sparked unease within the state Education Department, which oversees Victoria's 1500-plus public schools, and renewed concerns from the federal workplace watchdog about exploitation in the cleaning industry in which fierce competition for contracts is perpetually tightening profit margins.

The Building Services Contractors Association of Australia, a peak body for the sector, complains that underpayment also creates an unfair competitive environment.

"Our members are telling us that in government schools, reputable companies are being undercut on the price. It's becoming impossible for reputable companies who meet all their legal obligations to the schools and to the cleaners to compete," the association said in a statement.

The most troubling case uncovered has been a cleaner working across two public schools in the city's western suburbs.

An analysis of his pay records and actual hours worked shows he does at least 42.75 hours every week, but only gets paid for about 20 of them. His hourly rate of $24 becomes just $11.

"I'd have to be running to be able to do the work in the time they are telling me to do it," he said.

The 57-year-old veteran cleaner is paid to work at one school from about 3.30am to 5am, and the other in the afternoon, from 3.15pm to 6pm.

But to get everything done, he has to stay back at the first school more than an hour after his morning shift is supposed to end; then go straight to the other school to put in 45 minutes of work he couldn't finish from the previous night. About 9am, he goes home to try to catch some sleep before the afternoon shift begins.

And on weekends, he returns to both schools to work for another five hours between them, to empty all the outside wheelie bins and clean and dust down every table in the classrooms.

"There is no way I can fit them all in, so they have to get done on the weekend," he said.

"It's either that or they don't get done at all, and the company loses the contract and I lose my job."

Last year, 60 per cent of public-school cleaners surveyed by the cleaners' union were receiving below-award pay and conditions. There were "systemic award breaches" identified at nine out of 14 schools visited in just one week, according to a report handed to the Andrews government last December.

"A vicious price war has intensified in government-school cleaning," the union report said.

"Fly-by-night operators have flooded the market and pushed out reputable contractors. Many cleaners ... feel exploited and abused but are too fearful of losing their jobs to speak out."

Mr Barrientos is a dark-haired and soft-spoken refugee from El Salvador, who spent years as part of the large, and largely unnoticed, workforce of public-school cleaners.

"We were told, 'Don't get sick ... If I lose the contract, you'll lose your pay'," he recalled.

Both he and his wife worked for Ramos Cleaning Services at Epping Primary School, where they were paid $21 an hour to work daily three-hour shifts from Monday to Friday.

"We were paid for three hours a day, but we always had to stay much longer, and every Thursday we had to buff all the floors, which was a very big job, and took at least an extra hour," Mr Barrientos said.

"Then on Fridays we had to sweep, mop and vacuum the school hall, because it was rented out on weekends. That's another two hours."

Eventually, their employer lost the contract when another company came in and quoted a price that was 30 per cent cheaper, Mr Barrientos said, and he and his wife lost their jobs.

Ramos Cleaning Services director Alejandro Ramos last week denied that his former employees were expected to work longer than the hours they were paid for.

"That's not true; they never worked more," he said.

"I have 26 schools. I always try to put the right price to cover the cleaning that needs to be done."

Mr Ramos said the contract-cleaning industry had become unmistakably more competitive in recent years.

"Yes, it's very competitive, but so is everything," he said.

The national workplace watchdog, the Fair Work Ombudsman, said the cleaning industry was plagued by cases of underpayment and unlawful contracting.

Ombudsman Natalie James said there was a "race to the bottom" mentality in the industry, which had led to unsustainable business models, irresponsible contracting and unreasonably low contract prices.

A spokesman from the Ombudsman's office said Australia's cleaning staff were often "vulnerable workers at the bottom of complex supply chains": "Whilst we acknowledge that competitive tendering and tight profit margins can compromise the ability of some cleaning businesses to meet their compliance obligations, employers cannot look to cut costs by undercutting and ignoring minimum wage rates."

United Voice state secretary Jess Walsh said the union was aware of cases in which public-school cleaning staff were being illegally hired as independent contractors for $8 an hour, cash in hand.

"That's a third of what they're meant to get," she said. "When we ask who's employing them, they say it's some guy they've never met on the other end of a mobile phone."

Ms Walsh said cleaners, already among the lowest-paid workers in Australia, deserved "at least the legal minimum wage" and to be treated with some "basic respect".

"All of this is not lawful and it's not right," she said.

The Andrews government has been focusing heavily on the contract-cleaning industry amid rising concerns about workplace mistreatment.

It is in the process of bringing in controversial policies including strict regulations for labour-hire operators and allowing cleaners to carry their long-service leave with them from one employer to the next, so they don't miss out when contracts change hands.

Questions are being raised in Labor circles about why the government appears less prepared to act on the mounting evidence of cleaners being underpaid and exploited "in its own schools".

In response to the latest revelations, the Education Department said allegations of cleaners being underpaid were concerning, and urged affected workers to contact the department's cleaning contracts unit to investigate.

The department said the hiring of cleaning contractors was a school-based decision made by school councils. Principals were expected to review pricing structures contained in tenders to ensure quoted prices reflected the likely cost of service, including wage rates and expected time.