Lowering the cost of train fares to Sydney Airport would make little difference to heavy road congestion in the area, the state's transport bureaucrats say.

The relatively expensive train fares to the Domestic and International stations have become a money spinner for the O'Farrell government, which banked about $50 million last financial year from a profit-sharing agreement with the owner of the private stations.

Tourism and public transport advocates say the government should now start to reduce the $12.30 add-on cost of travelling by rail to the airport, in part to encourage more people to catch public transport.

But at a parliamentary hearing on Monday, bureaucrats from Transport for NSW said they did not think it would be worth the cost.

"From a transport planning perspective I would not abolish the station access fee," Carolyn McNally, the deputy director general in charge of planning at Transport for NSW, said.