Labor's home insulation program was flawed from the start, but no amount of regulation could have eliminated serious safety risks, former minister Greg Combet says.

Combet, who oversaw the scheme's closure, is giving evidence at a royal commission investigating whether four deaths under the program could have been avoided.

The former Labor frontbencher said that while there were serious flaws with the program's design, it was impossible to stop people from flouting workplace safety laws.

"In this instance respect for occupational health and safety laws is extremely important and there were clearly failings in that regard," he told the inquiry in Brisbane on Friday.

"And no amount of regulation from Canberra or program design can ultimately completely eliminate that risk."

Matthew Fuller, Rueben Barnes, Mitchell Sweeney and Marcus Wilson lost their lives installing insulation under the program.

Combet said he was genuinely upset by their deaths in a program that invited unscrupulous operators.

He said it was too easy for operators to rort the system as there was no way the government could check whether a home had been insulated.

This was ultimately why the Rudd government abandoned plans for another insulation program.

"At every turn, when we were contemplating a change in policy ... we received anecdotal reports of new ways people were trying to rort the program, including after it had been closed," he said.

"I just felt that in contemplating a new scheme, the risks were too substantial and there were clearly some unscrupulous people in the community who had been involved under the home insulation program who were very clever [at defrauding]."

Combet told the inquiry of the fear he felt about the prospect of fatal fires linked to the program.

"I lived in constant fear and concern there may be further house fires," he said.

He said he didn't want further injuries and was also mindful that a lot of cash "had gone out the door" under the program, which had exceeded its budget.

Meanwhile, the commissioner, Ian Hanger, QC, has hinted that former environment minister Peter Garrett may reappear at the inquiry after former prime minister Kevin Rudd was allowed to refer to secret cabinet processes in his evidence.

"Something that occurs to me is that it may be necessary to give Mr Garrett the chance, for example to say 'well, let me make this ... clear that what Mr Rudd said about this current thing was not correct'," Hanger said.

"So there are ramifications about the abandonment of cabinet privilege."

The inquiry continues.