Without a warrant, agencies including the police, Centrelink, the Taxation Office and even local councils have the ability to request access to personal information kept on databases linked to the card when investigating crimes like murders, welfare fraud, tax evasion or even littering.

Already in Queensland and Victoria, authorities have asked transport departments for access to records kept on smartcard systems for their investigations.

Since 2006, Queensland Go Card smartcards have been accessed 10,966 times. The majority of requests were made by police with most approved.

However, Queensland's public transport agency Translink said it had received one request for data from the Office of State Revenue, another request from Queensland Treasury, and two requests from the Department of Immigration and Border Protection in the past 12 months, although these were refused.

Since myki was rolled out in Victoria, its records have been accessed 274 times by Victoria Police and the Australian Federal Police.

NSW Police have yet to exercise the powers given to them as the new Opal card system continues to be rolled out across the transport network.

But they now have the power to follow the digital footprint for every train, bus or ferry journey taken by Opal card users.

"I think it's inevitable that law enforcement will request access to Opal records," NSW Council for Civil Liberties spokesman Stephen Blanks said.

"There's probably more than 100 agencies that have law enforcement powers which could have access to Opal records under its privacy policy. It's everyone from the RSPCA through to environmental protection agencies and even local councils."

Agencies like Centrelink, ATO and local councils can request the information because they are all classified as law enforcement agencies, and the Opal privacy policy doesn't define which law enforcement agency aren't permitted access. Whether they get access approved is at the discretion of Transport for NSW.

The term law enforcement is used in an act to do with telecommunications, which has led to Wyndham council in Victoria accessing mobile phone data and email records to chase residents for unauthorised advertising, unregistered pets and illegal littering. Bankstown council in NSW was also found using the powers to chase litterers.

Mr Blanks said he was concerned that no warrant was required to access records and people "not even subject to any investigation" could be targeted.

"Traditionally law enforcement has only had access to private records through a warrant system where they have to get approval from a judge and demonstrate that it's reasonably necessary for the warrant to be issued," Mr Blanks said.

"This [Opal] privacy policy ... allows disclosure of information by Transport for NSW without any warrant being issued and it's a serious weakness in the privacy policy."

When Fairfax’s Brisbane Times website revealed Queensland police were accessing public transport smartcard information in 2010, it found the records were being used not only to pinpoint the movements of criminal suspects but also potential witnesses.

Back then, alarmed civil libertarians recommended people concerned about their movements being tracked should remove their name and addresses from their card.

Although Mr Blanks conceded a warrant system would slow things down for police, he believed it was necessary to ensure accountability.

"Of course it would slow things ... but that's an appropriate way of managing access to data of this kind," Mr Blanks said.

The NSW Privacy Commissioner, Elizabeth Coombs, agreed a warrant process as an oversight mechanism was "very valuable" to provide proper accountability, but didn't go as far as saying the process was necessary for accessing Opal records.

According to Transport for NSW, Opal smartcard travel histories are stored electronically for 18 months before the data is then "decoupled from other personal information" and archived offline for seven years in accordance with state record regulations.

Since the initial launch in December 2012, more than 360,000 Opal smartcards have been issued and 20 million journeys taken with them. 

At present, all Opal smartcard users are required to register their smartcard and use their name. But this is expected to change once unregistered cards become available "in coming months", Transport for NSW said.

According to Transport for NSW, unregistered cards will allow customers to travel anonymously, with value able to be added using cash if the user doesn't want to link their credit card.

But despite being labelled "anonymous", unregistered cards could still be linked to people through other means, like surveillance cameras, the Opal privacy policy states.

Independent consumer advocate Christopher Zinn said that consumers often signed away their privacy without checking terms and conditions.

But he said people would likely be sympathetic to allowing police to access their Opal records if it helped solve a crime.

"... but it has to be accountable and it has to be transparent," Mr Zinn said.

Despite this, he said if agencies other than federal and state police began accessing records then consumers should be told about it.

"I think if the police are using such records to solve crimes which can be deemed as serious that's fine. But if it’s councils using it to catch errant dogs that’s another thing."

He also said that Transport for NSW should begin reporting publicly on the amount of records accessed.

Transport for NSW confirmed that discussions about Opal with "key" state agencies - including NSW Police and the NSW Information and Privacy Commission - occurred before its rollout. It pointed to its privacy policy, which states how information may be disclosed to third-parties and agencies.

It informs customers that Transport for NSW may disclose personal information or travel history to a law enforcement agency that is necessary for law enforcement purposes, for the investigation of an offence, for the enforcement of criminal law or to assist in locating a missing person. The public transport department may also disclose personal information to third parties in response to a warrant, summons or subpoena, it states.

NSW Police declined to comment.

SMH