A third of 729 phone scams reported in July were successful, resulting in the loss of HK$85.4 million (US$11 million).
Most of the victims were women aged 50 and under, Hong Kong police officials said.
The figures compare with 200 telephone fraud cases in the first six months of the year involving combined losses of HK$26.79 million, according to Ming Pao Daily.
Four cases were reported in the first half of 2014.
The latest scams involve people posing as mainland officials or police officers and courier company staff.
They would conspire to target Hong Kong parcel senders with bogus claims about illegal items found in the shipment and offering to fix the problem for a fee, Lo Mung-hung, director of the police crime and security unit, told a press conference Thursday.
In one case, a teenage girl was persuaded by swindlers to hole herself up in a hotel room while they contacted her parents to demand ransom.
She was later found by the police.
Lo said the Hong Kong police are working with their counterparts in the mainland and ramping up a public information campaign to fight scammers.
Meanwhile, he warned the public to stay calm when they receive suspicious telephone calls and to report them to the authorities.
Public security officials in Hong Kong and mainland China will never ask anyone for money or personal details, Lo said.
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