Prime Minister Tony Abbott will keep his job despite getting the selling of the budget wrong, former federal treasurer Peter Costello says.

A poor public reaction to the budget would not open the door for former opposition leader Malcolm Turnbull to challenge for the leadership, Mr Costello said.

“The Liberal Party does have the cult of the leader about it. Once you become the leader you’re almost untouchable,” Mr Costello told a leadership forum in Melbourne on Tuesday.

Mr Costello, as someone who spent “10 years denying” he was angling for the leadership, said he thought Mr Turnbull did not want to be seen to be undermining Mr Abbott.

He said the government was getting battered because it had not explained well enough the reasons for increasing taxes and cutting expenditure.

“If you just announce measures that’s one thing, but you’ve got to have a coherent story behind it,” Mr Costello said.

“That’s where I think the government’s got to go.”

Mr Costello said he would have focused on Labor’s “six budgets, six deficits”, with more to come, if he had to sell the budget.

He said the $7 Medicare co-payment was a good move as it put a “price signal” on a government service, but he said increasing marginal income tax rates would not help the economy grow.

“This was just a step backwards,” he said.

On the ever-present question of increasing the GST, the man who introduced it said he would only look at an increase if other taxes we cut or abolished.

“If the purpose of increasing the GST is just to give governments more to spend, there’s no point in doing it,” Mr Costello said.