The NSW Homicide Squad is investigating the 21 deaths and hundreds of coronavirus infections connected to the ill-fated Ruby Princess cruise ship.

They're working to determine if the ship's management, crew or operator were criminally negligent in allowing thousands of passengers to disembark in Sydney on March 19, despite others on board showing symptoms of COVID-19.

But could they actually be held liable for deaths which occurred after passengers left the ship — and even face murder or manslaughter charges?

If that were to happen, a key legal issue would be causation.

A recent High Court decision involving a violent home invasion is among the cases that may provide some insight into how causation is determined.

What happened in that case?

In 2013, Alexander Kormilets was bashed in his apartment in Redfern, suffering injuries to his brain, face, kidneys and chest.

The 78-year-old was hospitalised for four months, but didn't fully recover. The once independent and active man was left unable to care for himself properly, and moved to a high-care facility.

Eight months after the assault, Mr Kormilets fell out of bed, fracturing part of his left femur, the main bone in his thigh.

His family was told that an operation would likely save his life, and without one he'd probably die.

Thinking about his quality of life, which wasn't good at that point, they decided not to go ahead with the surgery. Mr Kormilets died in hospital not long after the fall.

William Rodney Swan, the man who had attacked him, was then charged with his murder.

Cause and effect

Swan took his case all the way to the High Court, but ultimately wound up in jail. ( ABC News: Gregory Nelson )

In the trial, the Crown argued that Swan's bashing of Mr Kormilets caused his later death.

"While there can be more than one cause, if the cause that the defendant is responsible for — here the serious assault that occurred in Mr Kormilets' home — continues to be an operating or substantial cause at the time of the death, then it's open to the jury to find the defendant liable for that death," explains Arlie Loughnan, a criminal law expert at the University of Sydney.

In other words, Mr Kormilets wouldn't have fallen out of bed, and then died, if it wasn't for the injuries he suffered in the home invasion months earlier.

The jury agreed — Swan was convicted of murder and handed a lengthy prison sentence.

Swan then appealed to the High Court of Australia. His argument can essentially be paraphrased as: "The family's decision to not proceed with surgery is what killed Mr Kormilets — not me."

The High Court unanimously rejected this, finding that the family's decision did not break the causation of the crime.

"They said that it was open to the jury to conclude that even though surgery on Mr Kormilets would reasonably be expected to save his life, his low quality of life at that point meant that the decision not to operate was an understandable one," Professor Loughnan says.

"So even though the man died eight months after the assault in his home, the assault itself remained what is legally required for liability here: a substantial cause of the death."

Have there been other similar causation cases?

Yes. Professor Loughnan says one of the most famous cases in this area is a decision called Blaue.

In that case, a woman who was assaulted died after she refused to have a life-saving blood transfusion, because she was a Jehovah's Witness.

"The defendant argued that the victim's actions in declining a blood transfusion constituted a break in the chain of causation — a stop, if you like, from his responsibility or his liability," Professor Loughnan says.

But the court "said that it did not lie in the mouth of the defendant to say what was reasonable on the part of the victim", and he was found guilty.

You can be found guilty of a homicide offence even if intervening medical treatment decisions come into play. ( ABC News: Robert Koenig Luck )

There is also the Royale case, involving a woman who fell off a balcony and died.

It was unclear whether she was deliberately pushed or if she fell accidentally. But ultimately it didn't matter, because it was clear that her violent partner had assaulted her and chased her onto the balcony.

That meant he could be found responsible for whatever happened on the balcony — in the same way Swan was responsible for Mr Kormilets' death, irrespective of any decision by the family.

These cases show you can still be found guilty of a homicide offence if there is a time lag between the actions of an accused and a death — and also if intervening medical treatment decisions come into play.

What about cases involving communicable diseases?

Health Minister Greg Hunt has warned that the deliberate transmission of COVID-19 could be punishable by a lifetime prison sentence.

There have been successful convictions for grievous bodily harm, manslaughter and murder involving communicable diseases, in particular the transmission of HIV.

These cases often involve people who had sex with a partner, and either deliberately kept their HIV-positive status from them, or secretly took off a condom and infected them.

"It is possible for a communicable disease to be the basis of an assault or an murder charge," Professor Loughnan says.

"That's because a way of committing murder or assault is by reference to grievous bodily harm — that's the legal expression for really serious harm. If a defendant transmits a disease to a victim, that can be one form of grievous bodily harm."

So what does all this mean for the Ruby Princess?

Almost 2,700 passengers were allowed to disembark while the ship was docked at Circular Quay. ( Getty: James D Morgan )

In the case of the Ruby Princess, the question will be what senior employees, crew or the corporation itself knew about the health of the passengers, what actions were taken, and what they did or didn't tell Australian authorities.

Then, causation could come into play.

"The information we have about the virus is developing quickly. So we'd have to think back to the moments of departure of the cruise liner, its docking back in Sydney and then the decisions about release of the passengers on to the shore," Professor Loughnan says.

Homicide detectives have already spent a month trying to determine what was known about potential coronavirus cases on board the ship before it was allowed to dock.

A Princess Cruises has previously told the ABC that all disembarking passengers, regardless of where they came from, were told the current requirements at the time, which were to go directly home and undertake 14 days of self-isolation.

"As we have stated many times, Ruby Princess followed the federal and state health clearance processes to the letter."

Professor Loughnan says it would be "very challenging" to bring a corporate criminal liability case against Princess Cruises, or its operator Carnival Australia.

"It can be very difficult to charge a corporation with a homicide, although it's possible under common law, because it's necessary for very senior people in the organisation to have the relevant knowledge. So it's a question of what they knew," she says.

Those people would need to be senior enough to be viewed as the "directing mind and will of the company" in order for the corporation to be liable.

Professor Loughnan says it is also possible that charges could be brought against an individual.

"The type of individuals who could be liable as we could imagine might be the ship's captain, senior crew, or perhaps management members of the ship's operator," she says.

But she cautions: "We must remember that the standard in criminal law of negligence is gross negligence. It has to be very serious negligence, very serious failure to live up to the standard of the reasonable person."

Negligence doesn't necessarily mean convictions

For a legal example of this involving ships, we can look to the 1987 Zeebrugge ferry disaster, where a cross-channel passenger ferry sank in Belgium, killing about 200 people.

The MS Herald of Free Enterprise capsized moments after leaving port with its bow doors not yet fully closed — which allowed thousands of gallons of water to flood in.

The ferry was towed into the harbour of Vlissingen in the Netherlands after salvage. ( Wikipedia: Archief Ranter, CC BY-SA 1.0 )

"The inquiries that followed to the disaster found that had been a practice of ferries at that time, because it meant that they could depart more quickly," Professor Loughnan says.

"But the subsequent inquiries found that was the cause of the disaster and also that it was a sign that the corporation and its management had had been negligent."

Charges were laid against the corporation, but prosecutions ultimately did not proceed.

"The prosecution was suspended for lack of evidence against the most senior members of the corporation," Professor Loughnan says.

"As we're saying, it's necessary to find the most senior members of an organisation have the requisite mental state for the charge in order to convict to corporations. It's not enough that more junior employees ... had acted negligently.

"And while there was very serious criticism levied at the corporation and its management as a result of that disaster, it didn't result in criminal convictions."

When asked about who made the decision to let passengers off in Sydney, NSW Premier Gladys Berejiklian said "all of us have to take responsibility".

But Australian Border Force Commissioner Michael Outram claimed NSW Health had given the green light for passengers to disembark.

Professor Loughnan says we're still in very early stages of the investigation into the Ruby Princess, and it remains to be seen what — if any — charges might be brought.

The Ruby Princess left Australian waters on April 23, five weeks after it was allowed to dock in Sydney.

The events surrounding its arrival are also the subject of a Special Commission of Inquiry in NSW.