Gull New Zealand says it's now talked to all its operators about docking staff for customer drive-off, and has discovered 29 people have been affected.

The cases are across five independent operators, which Gull has instructed to pay the workers back.

The company says reimbursement is underway, but as Gull doesn't employ the staff and therefore doesn't have their bank account details, it's being done via the operator.

General Manager Dave Bodger says by the end of today, 20 of the 29 will have their money back.

In a Facebook post, he says Gull's working to ensure this behaviour is not repeated.

There's been considerable flak over the practice, which is now being investigated across all petrol station brands by the Ministry of Business, Innovation and Employment's Labour Inspectorate.