Ryan Park, who is also Member for Keira, says the government has netted millions because the cards recharge when the balance falls to $10, meaning the remaining amount can never be spent.

‘‘Opal card users who have activated the ‘auto top-up’ option are essentially being charged a hidden $10 fee for the privilege of catching their train, bus or ferry,’’ Mr Park said.  

‘‘The Baird government has been encouraging commuters to use the ‘auto top-up’ feature on their Opal card so they are never caught with a low balance – but the reality is that the government is raking in millions in a devious cash grab.’’

A Transport for NSW spokeswoman said the minimum reset levels of $10 for adult and $5 for concession customers was to ‘‘ensure there is always enough value on the Opal card for these customers to undertake a single trip anywhere’’ – including an $8.30 adult train fare or $4.15 concession fare between the Illawarra and Sydney CBD.

‘‘It is not correct to state that a card balance does not get below $10 on Opal with auto top up,’’ the spokeswoman said. ‘‘A customer travelling from Wollongong to Central with a starting balance of $11 will get the fare of $8.30 deducted as they tap on, with a sum of $2.70 remaining, and then the auto top-up value will be added on.’’

Mr Park said at least half of the two million Opal cards being used were set to automatically top up and commuters needed to know if the money collected went back into public transport improvements or to government coffers.

The spokeswoman said when a customer tops up their card, by any means, the funds were securely held by the Commonwealth Bank – part of the consortium delivering Opal – and deducted by Transport for NSW when they travel.

‘‘Any interest earned on the funds is used to contribute to the costs of operating Opal and the electronic ticketing system, which is contracted to cost $1.4billion up to 2025,’’ the spokeswoman said.