Shirley said in imposing the levy the council and government showed "absolutely zero understanding of the retail fuel market" or how prices were "manipulated and massaged" around the regions.
While Warburton himself did not believe there was clear evidence of Auckland's regional fuel tax being spread to other regions yet, he said the numbers raised some questions.
"There does appear to have been some increase in prices in Canterbury and Otago regions. While we would expect this to happen if the tax was being spread from Auckland to those regions, it's also consistent with any number of regional factors that might cause prices to rise."
He also pointed out that Auckland's prices are not behaving differently from regions like Bay of Plenty and Waikato, suggesting that spreading had not occurred.
However, Shirley said "of course" the cost of the fuel tax was being spread to the regions "that's how markets work in New Zealand", it "highlights the nonsense of trying impose a ring-fenced regional fuel tax through a national system."