The Federal Government says there is "no way" the Opposition will be able to pay for the new roads it has promised.
The Coalition has pledged $4 billion for major projects in Sydney, Melbourne and Brisbane, and has promised to fix road infrastructure in some regional areas.
Opposition Leader Tony Abbott says if he wins Government he will begin construction on a major Sydney road project within a year.
Mr Abbott has already promised $1.5 billion to build the M4 East in the city's west if he is elected.
He says the road is critical to boosting the nation's productivity.
"This is the most obvious missing link," he said.
"Yes, we've got to get rid of the carbon tax, yes we've got to stop the boats, but by god let's see some cranes over Sydney again." However, Transport Minister Anthony Albanese, who announced funding for a large project in Darwin this morning, dismissed the plan as a "thought bubble".
Mr Albanese announced $200 million for project in Darwin and remote parts of the Top End.
But he says an incoming Coalition government would not be able to afford similar projects if it scraps the carbon tax.
"Tony Abbott has said he would get rid of the Minerals Resource Rent Tax.
That means getting rid of the $6 billion regional infrastructure fund," he said.
"[That means] less money for regional roads - it's as simple as that." Mr Albanese called on the Opposition to release detailed costings of its plans.