The vast majority of revenue lost from passengers disposing of cards with negative balances occurred at the airport stations.

The cost of catching the train to the airport is the most expensive on the city's rail network. A one-way journey from central Sydney to the domestic or international terminal costs an adult passenger $18.70 during peak hours due to a so-called station access fee.

However, train passengers can enter other stations on the network with a minimum balance on an Adult Opal card of just $3.46 during peak periods, or $2.42 at other times.

Transport Minister Andrew Constance said installing the technology to stop fare theft would enable the government to invest more into public transport.

"It’s important everyone pays their way, and this technology will ensure that," he said.

"We have Opal machines on both sides of the gates and also offer the convenience of tapping on and off with your credit or debit card."

A recent expansion of so-called contactless payments using credit and debit cards to Sydney's rail network was also intended to help stem fare evasion.

Among other recent measures to prevent fare evasion, passengers buying Opal cards at the airport stations need a minimum of $35 on their cards.  

Unlike in other major cities such as London and Hong Kong, NSW does not charge people for new Opal cards despite them costing more than $2 to produce and distribute.

The number of Opal cards with negative balances rose by 41 per cent to 1.1 million as at the end of June, from almost 777,000 a year earlier. Revenue from unregistered Opal cards with negative balances cannot be recovered unless passengers top up their cards.