The Transport Department said on Friday that staff negligence was to blame for a blunder in which 4,700 vehicles were charged the wrong tolls at the Western Harbour Crossing earlier this week.

Officials said a worker of the toll service provider failed to clear an old command, and another worker approved it by accident.

The error led to the new time-varying tolls being replaced by an old arrangement that charged vehicles a fixed rate from around 10am to 11am on Monday.

The department said it has received a report from the service provider, and has ordered the firm to make improvements.

It also noted that it will conduct a review of how the service provider operates the toll system, adding that its authorisation is now required to perform key procedures in the system.

In a statement, the toll service provider apologised to affected motorists, who have since been given a refund or an updated payment notice.

The company also said it’s taken disciplinary action against those responsible for the error.

"We pledge to continue improving the operation procedures, step up staff training and enhance our service quality, to avoid a repeat of such incidents," its statement read.