Employers would be able to offer extra shifts without penalties to part-time workers doing eight hours a day under a proposal from retailers.
The Australian Retailers Association has called for changes to the Morrison government's industrial relations omnibus bill.
Under the draft legislation, employees must work 16 hours a week to be eligible for the part-time flexibility arrangements.
ARA chief executive Paul Zahra said the group largely supported the bill, which is fiercely opposed by unions and Labor, but suggested a handful of tweaks.
The association's legal adviser Nick Tindley told a Senate inquiry into the legislation the 16-hour threshold should be halved.
"There's a significant cohort of employees who will miss out on that ability to let their employer know that they're willing to work additional hours at appropriate penalty rates," he said.
"We think that people working eight to 16 hours are more in need of that than those working 16 or above."
Minerals Council chief Tania Constable denied there was rampant use of labour hire in the mining industry after union members gave emotional testimony about job insecurity.
Ms Constable said just 11 per cent of mining workers were employed through labour hire, with the figure rising to between 30 and 40 per cent once contractors are included.
"This bill will help to encourage more investment in Australia's minerals industry," she said.
"It will create more highly skilled highly paid jobs, support regional communities and accelerate Australia's economic recovery."
Unions are at loggerheads with employers over provisions around greenfields agreements, which would allow for eight-year enterprise deals for major projects.
Electrical Trades Union member Robert Smith said eight-year enterprise agreements on major projects could stop workers from bargaining for improved conditions.
Mr Smith said companies were reluctant to acknowledge or rectify issues until they were addressed in negotiations.
"It was nicknamed the suicide roster," he told the hearing.
"We identified during the mining boom there was quite a high number of suicides in the construction sector."
After 14 deaths on one project, the issue was eventually tackled with shorter rosters, opportunities for workers to decompress, more days off and less time on call.
"If we have an eight-year agreement those things will never be addressed," Mr Smith said.
Australian Mines and Metals Association head of policy Tom Reid said the bill was a well-balanced package of improvements that would benefit the country.