Officials from the Australian Building and Construction Commission were grilled during Senate estimates on Wednesday about whether it would direct building contractors tendering for government work to take them down.

The regulator mandates all union logos and mottos be banned from clothing, property, or equipment supplied by an employer covered by its code on building sites.

"The poster can stay up, the logo can't ... is the short answer," Commissioner Stephen McBurney told the hearing.

A contractor found to have breached the rules would be given the opportunity to rectify it, officials say.

Senator Louise Pratt asked if a charity drive organised by a union would be allowed to carry a logo.

In response, Mr McBurney said the charity poster could go up, without the logo of union or employer association, if the messaging were to be placed on employer equipment.

"The union can do whatever they like, with charity posters and charity publication on their own accord on their own social media channels to their own members," he said.

"The code only applies to building contractors seeking to tender for Commonwealth work."

Senator Pratt asked Industrial Relations Assistant Minister Amanda Stoker what she made of the rules, as she supported freedom of speech.

Senator Stoker fired back that she believed in free speech but also supported "the workplace relations laws that exist in this country".

"Of course, any individual is entitled to raise any matter of public discussion anywhere they like really in their own time," she said.

"But when they're at work, when they are using property that's not theirs, there are limits."

The hearing was also told the ABCC had not prosecuted any entity over "sham contracting", which is where a worker is hired as an independent contractor instead as an employee.

Madeleine Jones, from the regulator, had previously said in estimates that "the bar to prove sham contracting is very high".

Meanwhile, the Fair Work Commission's general manager Murray Furlong could not rule out further appointments being made, with the prime minister just days away from calling the federal election.

"That's a matter for government," he told senators.

Mr Furlong said there was funding for 44 high level members - and one more vacancy for a commissioner after a resignation last week.

The Morrison government has come under fire for making a swathe of official appointments - some of them with Liberal Party links - as it approaches caretaker mode.

Australian Associated Press