Thrifty Sydney public transport users exploiting a ticketing loophole to score cheap weekly travel have been warned that the jig is up.

Transport Minister Andrew Constance has announced a crackdown on commuters "Opal running" between light rail stops and train stations to manipulate the ticketing system and costing up to $2 million in lost revenue.

Opal fare rules allow users to travel for free for the rest of the week after clocking up eight paid journeys.

As a result, vast numbers of commuters have been running, riding or skating back and forth between stations to "max out" their Opal cards with cheap trips to reap significant savings.

“What people are doing is tapping on, tapping off, attracting the eight journeys, and then travelling the rest of the week for free on longer trips," Mr Constance said.

He said the scam was seeing some 60,000 Opal transfers recorded at stations in and around Pyrmont on Mondays, which plummeted to just 100 by Friday.

The clear disparity in usage frequency appears to have been what first alerted Opal management to the rampant exploitation.

In a bid to stamp out the practice, the number of transfers needed to make a journey has jumped from three up to seven from today, making it much harder for users to fill up their cards.

The ruse, which had taken energetic card holders just 90 minutes to pull off, will now take up to six hours.

"We're going to clamp down on that loophole and we're going to stop people from doing it," Mr Constance said.

"It's a practice which is unacceptable, and as a result we're going to see people stopped from engaging in this behaviour on our network."

Mr Constance said funds lost via this loophole was money lost on improving services for travellers paying their fair fare.