During the official launch of its new "Tap to Pay" fare system, TransLink is warning users they may end up paying more if they don't do it right.

As of Tuesday, transit riders no longer need a Compass Card or exact change to access the system. They are now able to tap their credit card or use Apple Pay at the fare gates. The system also works with Google Pay and Samsung Pay.

The system designed by U.S.-based Cubic Transportation Systems is only set up for adult fares, and the only credit cards with contracts are Visa and Mastercard.

But those who aren't paying attention or are in the habit of tapping their card without taking it out of their wallet could end up being charged extra for transit trips. The sensors will charge the first card they detect, meaning if a credit card is picked up first, it could be charged instead of a pre-paid transit pass.

TransLink says multiple cards won't be charged at the same time, but it's best to the card out of the wallet beforehand.

If a different card is detected first when the rider taps out, they could end up paying what a user pays when they don't tap out – a fare for the entire transit system, not just the distance travelled – twice over.

Those using a method other than Compass Card will not see how much their card has been charged. The exact fare will be sent to the credit card company.

A user could end up paying as much as four times higher than the price of their fare if multiple cards are scanned. The card readers will automatically bill a user for travel through three zones, a fare of $9.90.

When a similar system was implemented by the same contractor in Chicago, a class-action lawsuit was filed against the makers of the card reader system by customers claiming they'd been double billed. 

TransLink said the agency consulted with cities including Chicago and London prior to Tuesday's roll-out, and that the double-billing issue wouldn't come up in Metro Vancouver because the scanners only detect one card at a time.

If the sensor detects more than one card at a time, the tap may be rejected, TransLink said.

In London, an automatic refund system was set up for "card clash."

Documents obtained by CTV News through the Freedom of Information Act showed refunds were granted refunds where customer service could infer from a rider's history of use that the charge was accidental.

When the system started, there were approximately 1,500 customers billed accidentally each day. Once people got used to the system, that number dropped to about 1,300 a day, or 0.5 per cent.

Locally, customers who see unexpected charges can contact Compass customer service at 604-398-2042.

Ahead of the launch, staff members were stationed across the SkyTrain network and messages played on trains warning users to tap just their card, not their entire wallet.

The transit provider also released videos with the same message, and put stickers on fare gates as a reminder.

Like Compass Card users, those using the alternate options are still required to tap out of the system, except on buses.

TransLink is the first provider in Canada to offer credit card and mobile wallet alternatives to traditional fares.