CANBERRA, AAP – Controversial workplace law reforms will almost certainly be delayed, with a key crossbench senator declaring negotiations will be impossible next week.

Industrial Relations Minister Christian Porter has taken time off after emphatically denying historical rape allegations.

Prime Minister Scott Morrison has backed his temporary replacement Michaelia Cash to lead negotiations while he is on mental health leave.

But independent senator Rex Patrick says Mr Porter needs to lead talks with the cross bench on the wide-ranging industrial relations omnibus bill.

“It’s impossible while he is absent from work,” Senator Patrick told the ABC on Wednesday.

“Asking Michaelia Cash to step in and do the negotiations where Christian Porter has had significant involvement in this negotiation from the get go is unreasonable.”

The government needs the support of at least three of the five crossbench senators to pass the bill next week, which will be the last time the upper house sits until the May budget.

“It will not be dealt with next sitting week, I can almost guarantee you that,” Senator Patrick said.

The South Australian won’t enter talks until the government has secured two crossbench votes because of constraints on his time as an independent.

He has also flagged concerns with the bill over concerns there is not enough recourse for casual workers who are refused a conversion to permanent roles.

“I have considerable concerns,” Senator Patrick said.

“There’s a lot of ground that would have to be covered before I would even come close to supporting it.”

Senator Patrick said he wouldn’t make an independent inquiry into the allegations against Mr Porter a condition of considering the legislation.

One Nation’s two Senate votes shape as crucial to the bill’s passage, with the party’s industrial relations spokesman Malcolm Roberts not opposed to it being debated next week.

Centre Alliance’s Stirling Griff and independent Jacqui Lambie have expressed concerns with parts of the reform package.

Labor and the Greens are fiercely opposed.

The coalition has dumped the most contentious part of the bill, which would have given the industrial umpire more scope to allow enterprise agreements that don’t meet the better off overall test.

The industrial relations omnibus bill makes changes to enterprise bargaining, wage theft penalties, long-term pay agreements on major projects and awards.