$65m sitting on lost, idle Opal cards

They're a necessity to travel on Sydney's public transport network, but 9News can reveal that more than $65 million is sitting idle on lost or dormant Opal cards.

And getting the money back can be tricky.

Figures obtained by 9News reveal about 12 million Opal cards have gone unused for a year or more, with an average of about $5.30 on each.

Commuter Kash Chauhan is one traveller who's amassed quite a collection.

"You forget your card at home, and you go get another one ... so then you keep building up," he said.

You can only get your cash back if your card is registered, and has a balance of at least $5.

If it's unregistered, you'll never see your money again.

Rachel Wheeler from Transport NSW said they encouraged customers to register their cards for peace of mind.