"I formally acquit you and you are free to go," said High Court judge Mr Justice Kevin Zervos.

His order for each of the six defendants yesterday came moments after prosecutors from the Department of Justice offered no evidence in each of the cases, and withdrew the drug trafficking charges.

The six were part of a group of nine, and the three other defendants remain on bail, pending their next court date.

The prosecution's decision to drop the charges came after months of reviewing evidence, all while the defence urged them not to pursue the cases because they were victims, not criminals.

"They have waited well over a year for this day to come," said barrister Michael Arthur of Gilt Chambers.

"Most of the time they have been in jail. In fact, two of the defendants have waited over 18 months.

The wheels of justice have turned very slowly, but my clients can now go home."

Californian woman Celia Eberhard, 68, and Brendan Toner, 62, from Northern Ireland, were those two defendants. Both were arrested in April 2014 and only released on bail in late September.

In court yesterday, Zervos was highly critical about why the six cases took so long to reach their conclusion.

"Never have the words been truer than now that justice delayed is justice denied," Zervos said. "The decision to terminate these six cases ... has taken too long."

But he also delivered a stern warning about the actions of the six, despite their acquittal.

"The quantity of Ice destined for Australia and New Zealand that you had in your possession was nearly 15kg. That would have harmed a lot of people. You, and others like you, have to vigilant and careful. It's not just a situation about yourself."

Zervos said he hoped the six had learned from their experiences and that the message had been heard, so people would not be "duped or to some degree with wilful blindness" into trafficking.

Before yesterday's acquittal, all six were on tenterhooks.

"None of us knows what's going on. We're on pins and needles and as the days go on, we're getting more and more nervous," said Eberhard a few weeks ago.

But as the judge's acquittal rang through High Court number 18 in Admiralty yesterday, there was a collective sigh of relief among the six.

Tears were shed, hugs were shared and a short round of applause was heard from the public benches.

Outside court, a crowd of supporters, media, lawyers and consular staff discussed the outcome.

Barrister Arthur described it as a landmark case in that the prosecuting authorities were "realising slowly" that people were falling victim to internet scams that turned them into unwitting drug mules.

Watch: Alleged drug smugglers cleared of charges in Hong Kong

"These scams typically target elderly or vulnerable people online and develop relationships with them, convincing them to travel overseas and take additional luggage back with them."

Earlier this year in April, prosecutors for the Department of Justice dropped its case against Elizabeth Kummerfeld, who was represented by Arthur.

"She was an 83-year-old woman whose circumstances were almost exactly the same as the six defendants who have now been released," he said.

"Like them, she had been tricked by a drug syndicate into coming to Hong Kong. Like them, she was a victim of a cunning internet scam. And like each of them she had always maintained her innocence."

Arthur, along with Gerry McCoy, SC, represented the six defendants under instructions from solicitors Christopher Morley of Morley Chow Seto and Cecilia Wong Lai-ying of Robinsons.

Arthur said yesterday's acquittal - of six alleged drug mules in six separate cases, because the prosecution had no evidence to show - was unprecedented.

The prosecution gave no reasons as to why they dropped the case. "But it would be a very fair comment that it is because they knew these defendants were innocent people caught up in the internet scams. Why else?" asked Arthur.

A spokesman for the Customs and Excise Department refused to answer questions about the six defendants. "Since the matter is still under ongoing legal proceedings, no further comment can be given in any detail."

Last month, the UN office of drugs and crime acknowledged the trend of vulnerable people being groomed as drug mules, echoing the position of the Australian Federal Police, who called it a growing trend.

The six people released yesterday have to wait over the weekend before they can get their passports back and book flights home, but they will all leave with their heads held high, according to Arthur.

"The six defendants came to Hong Kong as persons of good character and they will leave Hong Kong as persons of good character."