The Lamma IV ferry sank after crashing with another vessel on October 1, 2012, killing 31 adults and eight children in what became the Chinese city's deadliest maritime disaster since 1971.

A judge-led commission later found "a litany of errors" related to the Lamma IV, including a missing bulkhead door which contributed to the ferry sinking within two minutes.

The coroner's court held an inquest last year following a lawsuit brought by victim representatives.

Coroner Monica Chow said on Wednesday that the crash happened because of gross negligence by the coxswain from each vessel.

"I find each of the 39 deceased was killed unlawfully," she said.

Both coxswains were jailed in 2015 as part of a criminal case and completed their sentences years ago.

Coroners in Hong Kong look into the cause and circumstances of death but have no authority to punish culprits.

The inquest spanned more than 40 days and called 84 witnesses, including those responsible for designing, building and certifying the Lamma IV.

Ryan Tsui, whose brother and niece died in the sinking, previously said he followed the case with "incredulity" as he said witnesses dodged responsibility.

"Every mistake that could be made was made, and each of the parties was complacent," Tsui told AFP last year, referring to testimony by shipyard directors, inspectors and mariners.

Chow said on Wednesday that the passage of time led to "faded memories" and "inconsistencies" from witnesses.

Survivor Philip Chiu, who lost his sister in the tragedy, was among those in the courtroom for the ruling.

Aside from jailing the coxswains, Hong Kong courts previously handed prison sentences to two government ship inspectors who failed to ensure lifejacket compliance.

A government probe alleged misconduct by 17 Marine Department officers, but their names and the full report were not disclosed for privacy reasons.

The Marine Department told AFP last year it had amended marine safety laws and set up a dedicated team to "strictly review" vessel construction and modification.

After 2012, the average annual death toll in Hong Kong marine accidents fell from 7.6 to 3.3, though the average number of accidents per year stayed roughly the same.