JULIA Gillard's pledge to shield most Australians from carbon tax pain is under fire with cost blowout warnings on everything from groceries to cars, restaurant meals and department store goods.

Qantas and Virgin announced travellers would pay an extra average of $6-$7 for each domestic return flight under the tax. And the Food and Grocery Council warned bills could rise by three times the government estimate.

This would mean groceries increase by $120 a year, or $2 a week, rather than the official $40 prediction.

Special report: Carbon tax bottom line

It was also revealed Treasury had time only to model the economy-wide impact of the carbon tax on a $20 and $30 a tonne carbon tax, rather than the $23 rate to apply next year.

Household modelling was done at the $23 rate.

The Government defended Treasury forecasts saying they were robust and done by the same people who predicted impacts of the GST.

Ms Gillard said six million households would receive sufficient compensation to "come out square" or better off under a carbon tax, after expected price increases.

In the first trading day after the carbon tax was revealed, 25 Australian companies heavily exposed to the pollution tax had more than $7 billion wiped off their share price.

Local car makers will also feel the crunch, with industry sources estimating the cost of a family vehicle like the Aurion, Falcon or Commodore could rise by about $400.

On 3AW yesterday, Treasurer Wayne Swan was unable to say how the carbon tax would affect a Falcon.

"It would be very hard to make that sort of calculation at the moment," he said.

He also couldn't say what the price change for a can of tomatoes would be.

But he defended Treasury modelling.

"The team in the Treasury is the same team that modelled John Howard's emissions trading scheme and it's also the same team in the Treasury that modelled the GST, and ... they got all of that, actually, entirely correct," he told ABC radio.

The Master Builders Association says the price of a modest home will jump by $5000. Some economists are also predicting upward pressure on interest rates.

Grocery Council boss Kate Carnell was perplexed by the official food price rise calculation of 80c a week on a $200 basket of groceries.

"We don't get it at all. They are not in line with our understanding of costs in the supply chain," she said.

Restaurant and Catering Australia's Greg Parkes said soaring electricity bills would force restaurants to lift prices or reduce staff.

"Julia Gillard needs to get real," Mr Parkes said.

Originally published as PM told to 'get real' on tax pain