Sydney tourists will need to put $35 on new Opal travel cards to be able to use the Airport Link train line.

The new rules were put in place to close a loophole that Transport for NSW says cost taxpayers more than $7 million last financial year.

The minimum touch-on balance on trains is $3.54 during the peak and $2.47 off-peak.

It means tourists had been able to buy an Opal card with $10 balance at the airport and tap-off in negative balance, due to a $14.30 added airport fee.

They could then throw out the card and buy a new one to do the same for the return trip.

The new $35 minimum top-up is supposed to cover the cost of an adult return trip.

Some commuters have been using a loophole to travel in the negative to the airport to cut the cost of the fare. Photo: Photo: Reddit/ LionKing_P1

But single trips can cost $18.70 for adults during the peak.

A spokesperson said the minimum was set at $35 as the nearest rounded amount.

Customers can still buy a ticket for a single trip. The top-up minimum at the machine remains $10 for adults with existing cards, who can also top-up online with $10.

“The $35 minimum top-up is only for new cards at the airport stations,” a transport spokesperson told The New Daily.

There is nothing to stop those with existing cards from travelling to the airport in negative balance and throwing the card out.

The minimum top-up rule came into affect on September 24.

Travellers complained of the new rules on social media site Reddit earlier this week.

Many said it would discourage tourists from using public transport, especially if they only intended to use the Airport Link one-way.

“[It’s] literally cheaper to get a cab, not to mention Uber,” one said.

Another said they generally planned for a friend to drive them back to the airport for outbound flights, and the $35 minimum would be a burden.

“I usually take Airport Link train only one way.”