The scammers used deepfake technology to make it look like the company's UK-based chief financial officer was ordering a secret money transfer, according to the police.
The poor worker who fell for the trick was the only real person on the video conference, where everyone else was a computer-generated impostor.
The police said this was the first case of its kind in Hong Kong, and they refused to name the gullible employee or the firm.
Baron Chan Shun-ching, a top cop, said that before this, the fraudsters had only used one-on-one video calls to fool their victims.
He said: "This time, in a group chat, it turns out that everyone you see is a fake."
The scammers used public videos and photos of the staff to create realistic-looking deepfakes, which can mimic people's faces and voices.
Chan said the worker had been wary when he first got a message from the fake CFO in January, and thought it was a dodgy email.
But he changed his mind after the video call, where he recognised some of his colleagues and others.
Chan said the scammers did not talk to the worker much, but just told him what to do before hanging up.
The worker then followed their orders and sent £18 million to five Hong Kong bank accounts.
He only realised he had been duped a week later, when he checked with the company's head office.
Chan said the police had nicked six people so far in connection with this and other scams.
He also said they had found eight stolen Hong Kong ID cards that the scammers had used to fool face recognition software.
He said the scammers had used deepfakes to apply for 90 loans and 54 bank accounts between July and September last year.