Hong Kong, a common law jurisdiction, invites overseas judges to hear cases at its Court of Final Appeal and their presence has been seen as a rule of law barometer for the former British colony.

James Allsop, a former chief justice of the Federal Court of Australia, took up his Hong Kong appointment last year just as the apex court was hit with an exodus of foreign judges, some of whom cited Beijing's political crackdown as a reason for leaving.

In his second-ever judgment in Hong Kong, Allsop cast the swing vote that broke the deadlock between four local judges and handed the government a 3-2 win -- a rare outcome for a court not known for dissenting opinions.

At trial, Lam was found guilty of naming a police superintendent being investigated by the Independent Commission Against Corruption (ICAC).

Hong Kong law forbids revealing the identity of anyone being probed by the anti-corruption watchdog.

The superintendent was in charge of an investigation into a mob attack -- in which Lam was injured -- at a railway station in July 2019, at the height of Hong Kong's huge pro-democracy protests.

Police were accused of being slow to react that night, and the subsequent lack of arrests fuelled allegations of collusion with organised crime -- which the force repeatedly denied.

At the time, Lam cited "public interest" grounds to disclose that the ICAC was probing the police superintendent in charge of the case.

Lam was handed four months in jail, though his sentenced was overturned on appeal.

On Tuesday, Hong Kong's top court restored Lam's conviction and sentence on a technical point of legal interpretation.

Allsop wrote that the law forbidding disclosure was meant to protect the investigation's "integrity and efficacy" by not tipping off the people involved.

"If such were not prohibited, it might be thought that the protection of the integrity of the investigation actually being carried on was barely, or but weakly, protected by the (law)," Allsop added.

Beijing has tightened its grip on Hong Kong in the wake of the 2019 protests, imposing a national security law which officials said was needed to restore order.

Lam is already serving a sentence of nearly seven years, having been convicted last year in a separate case alongside other opposition figures under the security law.

In February, a judge added nearly three years to Lam's time behind bars after convicting him of rioting in relation to the 2019 mob attack -- despite the defence arguing that Lam had been a victim.

Five overseas judges have quit Hong Kong's top court without finishing their terms since the security law was enacted in 2020. Four others have not renewed their appointments.

Allsop is one of six remaining "overseas non-permanent judges" and his term runs until May 2027.