A legal expert says police have probably broken the law by obtaining the personal bank information of journalist Nicky Hager without a court order.

Court records show detectives investigating the hacking of Whale Oil blogger Cameron Slater's computer obtained nearly 10 months of transactions from Mr Hager's accounts from Westpac without the legal authority to do so.

University of Otago law professor Andrew Geddis said it seems police tried to take the easiest route by just asking Westpac for the information.

"They can do that, they've probably broken the laws by doing so, but it's just a little bit difficult considering the police believe that private information is available to them whether they think it's necessary just because they need it."

Mr Geddis said Westpac was not legally obliged to hand over the information to police, who did not have a court order to seek Mr Hager's private details.

"Customers of Westpac should now be aware that if the police come to Westpac and ask for information because police say there has been serious offending, Westpac will just roll over and give it."

In 2014 Nicky Hager wrote the book Dirty Politics, based in part on information obtained from a hacker known as Rawshark.