Victorian senator Ricky Muir has challenged Joe Hockey's comments about the impact of extra fuel tax on poor people, saying not everyone in rural areas can "hop on cows and ride into town".

The treasurer is under fire for his comment that low-income families would not be as hard-hit by the return of fuel tax indexation as the wealthy because "the poorest people either don't have cars or actually don't drive very far in many cases".

Senator Muir, from the Australian Motoring Enthusiasts Party, opposes the tax rise, especially given its impact on people in regional areas with further to drive and less access to buses and trains.

"I don't think (Mr Hockey has) ever lived in a rural, regional, or remote area where he may have been a school leaver, a job seeker, unemployed, low to mid-income where there's no public transport," he told the ABC.

"We can't all hop on cows and ride into town, I don't think."

Senator Muir told Fairfax radio on Thursday that he had often had to think twice about filling up his car, having lived in regional areas.

"Rural and regional will feel the pinch the most," he said.

The crossbench senator said he had met Mr Hockey to discuss the federal budget but "no deals or agreements were made".

He has also spoken with doctors who voiced concerns about the $7 GP co-payment.

"I'm not going to come out with a position yet ... but in its current form it is a concern," he said.