Australia's legal bill for defending its cigarette plain packaging legislation is set to hit $50 million as it battles to contain a case brought by tobacco giant Philip Morris before a tribunal in Singapore.

And that is just for the first stage. If in September the three-person extraterritorial tribunal decides Australia has a case to answer, the hearing will move on to substantive matters and the bills will become far bigger.

The West Australian newspaper revealed on Tuesday that former treasurer Wayne Swan was called to Singapore in February to give evidence for Australia in a secret hearing.

Among the witnesses called by Philip Morris has been former High Court judge Ian Callinan, who was quizzed about administrative law.