The Lamma IV ferry sank after colliding 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.

Chow only delivered part of her ruling and will continue on Thursday.

Some of the coroner's remarks proved divisive among the victims' families.

Alice Leung, who lost her brother in the tragedy, said she was "not surprised" by the finding that the 39 people were unlawfully killed.

But she said she was disappointed by Chow's conclusion that the missing bulkhead door was a "rational design decision" in accordance with shipbuilding standards of the time.

"From (the ruling) that I heard today it seemed nobody was found to be at fault," Leung said.

Ryan Tsui, whose brother and niece died in the sinking, said Wednesday's outcome was "not a victory".

He earlier criticised the testimony of shipyard directors, inspectors and mariners, saying all the parties dodged responsibility.

Leung and Tsui said they will decide on next steps after hearing the coroner's full reasons.

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.

Hong Kong's transport and logistics chief, Mable Chan, said the government will issue a response once the court completes its ruling.

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.