More than 40,000 passengers who were overcharged for trips booked using ride-hailing giant Grab's mobile app will be getting their money back.

The customers were overcharged during a 15-day period stretching from Nov 20 to Dec 4 because outdated Electronic Road Pricing (ERP) rates had been applied by the app when calculating fares for 60,787 trips, said the Land Transport Authority (LTA) and the Public Transport Council (PTC) in a joint statement on Dec 22.

The error affected 40,431 passengers, and the excess ERP charges collected ranged from $1 to $3 for most of these passengers, the statement added.

On Nov 15, LTA said ERP rates would fall by $1 at 10 locations during the December school holiday period from Nov 20 to Dec 31.

The two government agencies said the customers who booked trips using Grab gave feedback on being overcharged.

The statement said: "Having established that this was due to inaccurate ERP charges being applied to fares, LTA directed Grab to go through all their trip data to identify other trips where passengers might have been similarly overcharged, and to take relevant corrective measures."

It added that the agencies understand from Grab that it has updated the ERP rates in its system and has since taken action to reimburse all affected passengers.

In a statement on Dec 22, a Grab spokeswoman said a technical glitch resulted in a failure to update the ERP rates in its system, after LTA cut ERP rates by a dollar at 10 locations.

She said: "As a result, trips that passed through the affected gantries were mistakenly charged an additional $1. This impacted a very small percentage of our rides between Nov 20 and Dec 4."

She added that the ride-hailing firm has double-checked ERP rates for all gantries in its system, and implemented additional internal testing on top of existing processes to prevent similar incidents from happening.

She said Grab has contacted affected passengers through e-mail and its app in the evening on Dec 22 and informed them that they will receive refunds through their GrabPay wallets.

"We sincerely apologise to consumers for the inconvenience caused," she added.

In their joint statement, PTC and LTA said operators of ride-hail services are responsible for ensuring that accurate fares are charged to passengers, and added that the two agencies will investigate the incident.

Anyone who has been overcharged or encounters problems with their fares can lodge a report with the authorities, the statement added.

Passengers can lodge reports via the contact form at https://www.lta.gov.sg/feedback or through the e-service "Report Vehicle-Related Offences" at www.onemotoring.lta.gov.sg 

To facilitate investigations, passengers should provide information such as the vehicle's registration plate number, date, time and a brief account of the incident.

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This article was first published in The Straits Times. Permission required for reproduction.