Police are investigating after about 40,000 people lost at least 10 million baht as a result of unauthorised online transactions.

The commissioner of the Cyber Crime Investigation Bureau (CCIB), Pol Lt Gen Kornchai Klaiklung, said the police are in talks with the Bank of Thailand and the Thai Bankers' Association to find a solution.

"Initial checks found around 40,000 people have lost money," Kornchai said, before adding the majority of victims didn't realise their money was being withdrawn as the perpetrators generally charged small amounts -- albeit repeatedly.

Kornchai said such withdrawals are possible if the perpetrators get their hands on customers' bank card details, which are often stored by online retailers and websites.

He said the police suspect multiple gangs are responsible for the crimes.

Some gangs "phish" for their victims' details by sending out text messages which asked them to fill in their personal details in exchange for discounts and other perks, he said.

Meanwhile, some charges and withdrawals were made by store employees who stole their customers' card details when their victims were buying something, he said.

Many victims are sharing their experiences on a Facebook page called "Sharing Experience on Unaware Cash Withdrawals".

They questioned the security and reliability of Thai banks' because they received no alerts when the unauthorised withdrawals were made. Most victims only found out when they went to a bank to update their passbook.

One victim posted her passbook, which showed 13 withdrawals on Oct 11, each for 34.15 baht.

The Facebook account had more than 57,000 followers as of Monday.