Melissa Tang Shuk-nei, the group's chief forensic accountant, said the larger number of transactions involved in each case showed fraudsters were conjuring up more elaborate smokescreens.

"They create more items on the accounts to make the accounts appear legal," she said.

The forensic accounting group, which comprises six accountants, helps investigators determine which documents to confiscate, analyses complicated statements and helps interrogate suspects. Its members also testify as expert witnesses.

Senior forensic accountant Edmond Wong Hei-man said illegally obtained money is frequently separated into many small transactions.

"It involves more people, bank accounts or companies to create multilayered transactions," he said.

In one case, a clerk was jailed for seven years for falsifying more than 200 cheques, involving more than HK$10 million. "Each of [the cheques] involved several financial documents. We had to go through more than 2,000 accounting items," said Tang.

Tang said that with many companies expanding abroad, the group had to spend more time tracing overseas bank accounts and learning about other countries' regulations.

Some firms also invest in financial products overseas - often in the name of offshore companies - making investigation more difficult, she said.

The investigators sometimes use data analysing software but "more often, we use our own brains to trace the [suspicious] transactions", Tang added.