Tony Abbott likens persistent quizzing over his "plans" for the GST to going "round and round the mulberry bush".
Each day the opposition leader's denials get more emphatic, and each day Labor niggles away, saying he must come clean.
Abbott's branded Labor's attack as a "pathetic scare campaign".
"I want to make it absolutely crystal clear, there will be no change to the GST. Full stop, end of story," has now become a standard response.
Prime Minister Kevin Rudd's gone so far as to layout the impact from expanding the consumption tax to food and raising it to 12.5 per cent from 10 per cent - such as a 52-cent rise for a jar of Vegemite.
Even if the Liberal leader had the tiniest inclination to pursue changes in a first term of a coalition government, it seems extremely unlikely.
Given the ferocity with which Abbott pursued former prime minister Julia Gillard for introducing a carbon tax - when she said she wouldn't - he doesn't want to leave himself open to a similar attack.
Abbott argues that even if there was such a proposal, it would be monumental task.
All states and territories, and both federal houses - the House of Representatives and the Senate - would have to agree. Labor argues Abbott could simplify this by introducing new federal legislation.
Former coalition prime minister John Howard believes tax reform needs to be revisited.
His government introduced the GST in 2000 to provide funding for states and territories and enable them to eliminate less efficient state taxes.
But he says if negotiations with Labor and the Democrats party at the time hadn't "punched a hole" in the GST, the financial position of the states would be better than it is.
Last year's analysis by the Grattan Institute found widening the base of the GST to take in the current exemptions of health, education and fresh food would increase gross domestic product by $20 billion, allowing other taxes to be cut or abolished.
The coalition will have a taxation system review in government that would include the GST.
Any significant tax changes would be put to voters at the 2016 election.
The $50 billion consumption tax was not included in the Labor-sponsored Henry tax review, which was seen as a mistake by many tax experts given its importance as a revenue raiser for the states.
Even the Organisation for Economic Cooperation and Development (OECD) and the International Monetary Fund believe Australia should lift the GST rate or broaden its base, or both.
The OECD again urged Australia in May to "enhance efficiency" in housing taxation and lower the corporate tax rate through increasing the GST.
The Urban Development institute of Australia (UDIA) this week added its voice to such calls.
It says Tuesday's Pre-election Economic and Fiscal Outlook - an independent analysis by Treasury and Finance - highlighted the need for efficient and equitable taxes to boost investment and jobs growth, while reducing the taxation burden on the economy and provide budget stability.
UDIA national president Julie Katz said wasteful and inequitable taxes such as stamp duty on property purchases needed to be replaced by more efficient and broad-based taxes like the GST.
"Whilst both the ALP and the coalition have explicitly stated that broadening the base of the GST will not be addressed during the current election campaign, it is important that it remains on the reform agenda in the future," Katz says.
Yet GST-architect Costello is against increasing the impost.
"I think you would increase it and they'd (the states) just waste the money anyway," he says.