Young lawyers are pushing back against a culture of overwork in Australia’s top commercial firms, caused in part by the demands of the banking royal commission.

In the past three months, two top-tier firms, Gilbert + Tobin and King & Wood Mallesons, have been investigated by safe work authorities in New South Wales and Victoria, and advocacy groups say a culture of overwork and exhaustion runs through the industry.

The Legal Forecast, a not-for-profit group that provides support for students and early-career lawyers, has put out a position paper calling on firms to change the way they work.

Earlier this year, lawyers at the Melbourne office of King & Wood Mallesons told the Australian Financial Review young graduates were sleeping in the office rather than going home. In Sydney, Gilbert + Tobin has been warned by SafeWork NSW over insufficient rest breaks and potential drug use among workers.

In July, a source at a leading firm told the Australian Financial Review that “somebody is going to die” from the extra workload taken on by firms representing banks before the commission.

Alex Solo worked as a graduate at a top-tier firm from 2013 to 2017. He told Guardian Australia the stories of overwork would be familiar to any lawyer.

“Staying back late is part of the culture,” he said. “Leaving at 5pm is sort of laughed at. If you worked a late night, you wouldn’t go home. You don’t really sleep. You’d go home the next day and have a nap, but you’d work all through the evening.”

The long hours often exacerbated poor mental health issues within the profession, the position paper from the Legal Forecast said.

“Mental health issues are pervasive among the profession and fuelled by the widespread belief that a willingness to work well beyond the standard 38-hour work week is necessary to excel,” the group said.

Edwin Montoya Zorrilla, a spokesman for the Legal Forecast, said a lot of overwork was due to the system of billable hours, where firms charge clients for each hour their lawyers work.

The group is also campaigning for other solutions such as efficient use of automation and a rethinking of employment contracts, but billable hours were “the crux” of the problem, Zorrilla said.

“It prevents companies from innovating to perform more work in the same time because you are billing by time rather than outcome,” he said. “In other industries, like accounting, it has been superseded by other systems of billing.

“Low-skilled work is dumped on junior lawyers most of the time. Because their performance is measured around billable work, there’s really no limit to how much work that can be put on them. Their work is whatever is necessary for what the senior lawyer has to do.”

Solo agreed, saying it was a structural issue. “The billable hour system makes money the more that people work. There is always that pressure in the system to be working more and more hours.”

Both lawyers said the excessive workload of top firms did not begin with the royal commission, and also stretched beyond commercial firms to the bar and the courts.

“Everyone in the firm works pretty ridiculous hours. It’s not just the young lawyers, but it might be tougher on them,” Solo said. “It’s been bubbling beneath the surface for a long time.”

Solo has since founded his own law firm, Sprint Law, that does not use a billable hour system.

Zorrilla said speaking out about the culture was a career risk for young lawyers.

“We commend any individuals who are coming forth in the course of these investigations,” he said. “It takes a lot of courage to do so. It generates some amount of risk for people’s careers. Nobody wants to be seen as the one who can’t keep up with the demands of the firm.

“But it should not be incumbent on individuals to drive the change themselves. Pointing the finger at one or two companies is not going to lead to any widespread change. It needs to come from the top down.

“Every time this is brought up with someone senior at the firm, the response is that they are also under significant pressure. But you need to think about your grads in firms in more than just what they can provide in billable hours.”