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.
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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.
AAP