TREASURER Wayne Swan has responded to criticism of one of Australia's most controversial and complex taxes by proving the price of Weet-Bix has not risen overnight.

Speaking at Woolworths Nundah - opposite his north Brisbane electorate office - yesterday, Mr Swan said fears the tax would send the cost of groceries soaring had been exposed as "lies". And he had the shopping bags to prove it.

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On Friday evening, Mr Swan visited the supermarket and bought 11 items at a cost of $35.30. Yesterday morning, he bought the same items for $35.10.

His shopping included Weet-Bix, which Opposition Leader Tony Abbott claimed would cost more under the tax.

He also bought a $20 lamb roast, in addition to the 11 items, which Nationals Senator Barnaby Joyce has claimed would cost more than $100 under the tax.

Mr Swan said Sunday was the day "that Tony Abbott is going to get mugged by the truth".

Brandishing a box of Weet-Bix and the lamb roast, Mr Swan said: "Does anyone seriously think this (lamb) is going to go to $100 any time in the next month, two months, three months? Or that the price of Weet-Bix is going to double?

"Yes (the carbon tax), will have a modest impact on prices overall, but all of the scaremongering from Mr Abbott and the Liberals is disproved here by the items we're looking at."

Mr Swan said the average cost rise would be 0.7 per cent, with many households receiving financial assistance to cover the increase.

Mr Abbott has said the "price of everything" will go up under the tax and has pledged to repeal it, if elected, saying it was based on a "lie".

Hendra mother-of-two Sue Pearson said for many people the prospect of a new tax was "completely terrifying" but was not expecting to see a significant rise in her grocery bill.

"I'm not incredibly opposed to it (the tax), but I think the people that are creating the largest environmental problems should be paying the most for it," she said.

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