Double dissolution remains possible

Former prime minister John Howard says a double dissolution election remains a possibility and even with Senate voting reform, that would likely get more minor party senators elected.

Mr Howard, who celebrates 20 years since his election in 1996 at a dinner in parliament house on Wednesday, said the principal beneficiary of Senate voting reforms would probably be the Greens.

"That is why the Australian Greens are so strongly in favour," he told ABC television.

Mr Howard said it would be a good idea if only one voice spoke for the government on when the election would be held but that was a matter for the PM.

"A double dissolution is clearly a possibility," he said.

Mr Howard said there was clearly a case for either broadening the base of the GST or increasing the rate. As well, he said Australia would need to revisit industrial relations reform.

The former PM wasn't choosing favourites among his coalition successors.

"My view is that Tony Abbott had my strong support while he was there and now Malcolm Turnbull's there he will have my strong support," he said.

Mr Howard said his principle was to keep Labor in opposition.

"I remind all Liberal supporters of the validity of that operating principle," he said.

"It's always a good idea to remember that any government can be defeated. This idea that there is no unlosable or unwinnable election is no longer valid."