Woo said officers had sought help from their counterparts in Macau and Zhuhai in an effort to trace the “dirty” money and locate its recipients at the other end of the criminal chain. He said investigations were continuing and further arrests were possible.

Customs officers on Thursday last week arrested two drivers and seized HK$20 million after intercepting their vehicles – a seven-seater van and a tow truck – on the Hong Kong side of the mega bridge.

The arrests were made at about 7pm after a black recycling bag, packed with HK$20 million in HK$500 and HK$1,000 notes, was moved from the car to the truck.

The two men, aged 40 and 43, were among three suspected couriers accused of smuggling HK$166 million in 17 cash-smuggling operations since May, Woo said, adding the ill-gotten proceeds were taken to either Macau or Zhuhai, a neighbouring city in Guangdong province.

He said the biggest sum involved in a single delivery was HK$27 million.

Last Friday, customs officers raided the Kwun Tong shop of a money exchange firm, picking up its director and a staff member, both men aged 41. Officers seized about HK$300,000 from the shop.

The third suspected cash courier, 43, was detained on Monday when he turned up for an interview with officers at customs headquarters in North Point.

The five men, aged between 40 and 44, were arrested for money laundering – an offence that carries a maximum penalty of 14 years in jail.

Woo said he believed the five suspects were the members of a money-laundering syndicate that had been operating since May.

“We are still investigating the original source of the illegal proceeds,” he said.

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Customs officers began investigating the gang in May when the 40-year-old driver of the van declared he was carrying HK$3.8 million while exiting the city via the Hong Kong-Zhuhai-Macau Bridge control point on Lantau. Officers checked other declaration records and also identified the 43-year-old van driver.

Alice Ho Mei-ling, acting superintendent of the same bureau, said one of the two drivers declared he was carrying a total of HK$88 million when exiting the city at the customs control point on seven different occasions between May and August.

The other driver claimed to be carrying HK$78 million when he left via the same checkpoint 10 times over that period.

Both said the cash belonged to them and that the money was being taken to either Macau or Zhuhai, according to Ho.

“The huge amount of cash they declared at the control point while exiting the city was not commensurate with their income and financial backgrounds, and this aroused our suspicions and sparked the investigation,” she said.

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“One of the seven-seat car drivers lives in a public housing flat in Tin Shui Wai and has savings amounting to tens of thousands of dollars, while the other lives in a subdivided flat in Yuen Long and has a few hundreds of dollars in his bank account. Both men did not own any property,” Ho said.

“They carried nearly HK$170 million out of the city in three months and it was very suspicious.”

Further investigation led officers to identify the 43-year-old tow truck driver who was thought to have helped with the cash deliveries, Ho added.

During each delivery, bags carrying the money were moved from the seven-seater vehicles to the tow truck at a site about 2km away from the border checkpoint.

The tow truck carrying the cash continued the journey, while the cars returned to Hong Kong within an hour.

“We believe the tow truck was used to deliver the dirty money because it could come under less scrutiny and did not arouse suspicion from law enforcers while pretending to carry out legitimate car-towing business on the bridge,” a source said.

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Authorities do not believe the tow truck entered either Macau or Zhuhai with the cash, and instead handed it to another courier who then took the bags to their final destination.

“We believe each of the cash couriers involved were paid more than HK$10,000 in each cash delivery,” the source said.

The money exchange company was linked to the scheme by the investigation revealing that the alleged couriers were going there to collect cash.

The five suspects have been released on bail, pending further investigation.

In July, customs officers shut down a money-laundering syndicate that used cryptocurrency to process illegal funds totalling HK$1.2 billion, leading to the arrest of four people.

Customs in April arrested six people, including a couple and their son, on suspicion of laundering about HK$2.5 billion through 59 bank accounts over two years.

The city’s largest ever money-laundering case – involving HK$13.1 billion – came to light with the 2012 arrest of a 22-year-old mainland Chinese man.

He laundered the money through his bank accounts between August 2009 and April 2010, making 4,800 deposits over an eight-month period. In January 2013, he was sentenced to 10½ years in jail.