Nine people have been arrested over an alleged HK$6 billion money laundering operation which customs officers believe is their biggest bust ever.

Rita Li, head of customs' syndicate crimes investigation bureau, said on Wednesday that she suspects the alleged money laundering was organised from a flat in Hung Hom.

"The security of private housing estates is rather tight, passwords or resident cards are needed for entering and exiting...This made our investigation more difficult," she said.

During an operation earlier this month, officers seized about HK$3.9 million in cash, along with cheque books and company chops.

Officers suspect a gang opened a number of personal and company bank accounts and laundered some HK$6 billion via more than 7,600 transactions between January 2020 and December last year.

Yeung Yuk-man, from the financial investigation group of the bureau, said it's thought some of the alleged illicit funds were transferred to a nearby money exchange store.

"We do not rule out that someone brought the large amount of money with unknown sources back to the operational centre for further handling, and then divided the money among other members to be deposited over bank counters or through automated teller machines nearby," he said.

Yeung added that there was no evidence to suggest the money exchange store was complicit in the alleged money laundering.