Transport Minister Anthony Albanese insists Canberra cannot be Sydney's secondary airport because no other capital city in the world has one more than 100 kilometres away.
NSW Premier Barry O'Farrell says a second Sydney airport is unrealistic and wants Canberra's facility expanded and the two cities linked with high-speed rail.
But Mr Albanese says Sydney is the gateway to Australia.
Expecting foreign visitors to fly to Canberra and catch a train north was "laughable".
"It's like buying a ticket to New York and landing in Washington DC," he told reporters in Canberra on Monday.
Mr Albanese said Canberra, 280km south of Sydney, was too far away to be a viable option.
"There's nowhere in the world where an airport is used as a secondary airport more than 100km from the city," he said.
"There's nowhere in the world where you fly into a major global city and have to travel 300km to get to that city."
He said Australia needed aviation capacity.
Mr Albanese favours building a second airport at Wilton, on the south-western fringes of Sydney.
"This is the last opportunity we will have," he said.