Public transport passengers are illegally being slugged with MetroCard ticket recharge fees of up to $2 by some shops, which can add up to 20 per cent to their bills.

The debit cards allow passengers to frequently deposit money for multiple trips, which can be done through Transport Department offices or more than 300 private agents.

The department, under questioning, has confirmed some passengers are being charged an illegal fee of up to $2 and has warned providers to stop the practice.

The issue was first raised by the Labor MP for Wright, Blair Boyer, when constituents complained about one shop with Transport Department recharging rights that added the fee.

He said everyone should check their private recharge agent was not secretly adding $2 to their weekly bill.

“It’s concerning to see Adelaide Metro retailers taking advantage of public transport customers by choosing to apply these surcharges, over and above fees charged to them by their bank,’’ Mr Boyer said.

He has written to Transport Minister Stephan Knoll calling for an urgent audit into which retailers were applying the surcharges. He said those that were should be struck off the list of approved retailers.

How do Australia’s train networks compare to one another?

Many passengers who cannot afford to have large amounts of cash on their cards will recharge with the minimum of $10 several times a week.

A Transport Department spokesman said two agents had been the subject of complaints by passengers — one charging a $2 fee and the other 50c

“It should be noted MetroCard agents are permitted to recoup ‘reasonable costs’ as a surcharge for credit card purchases, as per ACCC regulations,” the spokesman said.

“Adelaide Metro has worked closely with the MetroCard agents in question to ensure they understand and operate within the ACCC’s regulations.

“DPTI will continue to address any known issues brought to our attention and will ensure the MetroCard agents in question are working within the relevant legislation.”

In 2017, after a more widespread problem, the Australian Competition and Consumer Commission ruled the charge on credit cards should be between 0.5 per cent and 3 per cent depending on the card.