AS INVESTIGATORS continue to probe the alleged murder of AFL coach Phil Walsh by his son Cy, the tragedy has shed light on an area of family violence we don’t often think about — when children are charged with turning violent against their parents.

And what happens next can be unthinkably devastating.

One parent is killed by their child every month in Australia, according to figures from the Australian Institute of Criminology.

A May 2015 report by the institute found parents were the third most frequent group of victims of family homicides, behind partners and children.

Between 2002-03 and 2011-12, there 134 parents were murdered by their children — a crime known as parricide. Ten incidents resulted in the deaths of both parents.

By way of comparison, 654 people were murdered by their domestic partner in the same period, and 238 children were killed by their parent or parents. There were 40 victims of sibling homicide between 2002 and 2012.

The majority of murdered parents (88 per cent) were killed in their family home, and in most cases between 6pm and midnight. Four were killed in someone else’s home; another four were killed in a health or mental care facility.

Sons were the most likely to murder a parent, with 80 per cent of offenders male. And in 73 per cent of cases, the victims were stabbed to death.

Samantha Bricknell, research manager at the Australian Institute of Criminology, said those factors — the sex of the offender, the location and the cause of death — meant in many ways parricides were generally statistically consistent with most other forms of homicide in Australia.

But a closer look at the statistics paints an even more grim picture of the crimes.

“While predominantly the offender is a male, and predominantly the victim is a male, we did find in earlier analysis published in 2003 the usual circumstantial thing is sons killing their fathers, and daughters killing their mothers,” Ms Bricknell said.

“We have to remember when we look at homicide more generically men are over-represented as victims ... but a son is potentially more likely to inflict damage when he’s up against another male figure.”

Most sons and daughters who kill their parents are aged 18 to 49, according to the institute’s statistics. Some are younger — and some are even older.

“They’re the ones that interest me, when they’re killing their very, very elderly parents and that certainly does occur,” Ms Bricknell said.

“There’s a lot of focus on that 18- to 34-year age group, but when you’re looking at 40-year-old men, or 70-year-old-men, who are killing their dads it’s much more unusual.

“We also don’t have a lot of multiple homicides in the sense that you have multiple victims or multiple offenders, but I think what’s interesting is ... you have a slightly higher proportion of multiple victims with parricides. And I think what the temptation is mum coming in and defending, and she’s also getting killed.”

An earlier report into family homicides by the AIC, released in 2003, found domestic arguments were the most common motive for killing a parent, where a motive had been established. Nine per cent of these murders were revenge killings.

Ms Bricknell said while the parricide could be motivated by abuse a child suffered by their parent, it was ultimately very difficult to determine the various motivations behind the killings.

“I think with any domestic homicide there’s a whole range of reasons that may be in place that charge that particular event, long-seated issues as well as spontaneous things,” she said.

“Because the pattern isn’t so different from (other kinds of) homicide I think we have to realise — and this is the tragedy of any kind of violence — a lot of it occurs for very banal reasons. And that’s why I don’t want to say that in every kind of parricide there must have been some sort of physical abuse, or some kind of traumatic event, that occurred to that particular individual to go and kill their parent.

“It might have just been a stupid argument that got out of control, and if drugs and alcohol were involved then that's going to be the case. And that’s really, for me, the tragedy of homicide. A lot of them — I wouldn’t say they could have been stopped — but it’s just the ridiculous combination of factors that led to that particular event occurring.”

And when they do occur, these crimes send shock waves through the community.

Sef Gonzales was just 20 when he murdered his parents, Teddy, 46, and Loiva, 43, and his sister Clodine, 18, in their north-eastern Sydney home in 2001. After a high-profile case, Gonzales was found guilty of the three murders and now serving three concurrent life sentences without parole.

More recently, in 2009, Antony Waterlow fatally stabbed his father, prominent art curator Nick Waterlow, 68, and his sister Chloe Heuston, 37, in Chloe’s home in eastern Sydney. Waterlow, who was diagnosed with paranoid schizophrenia, was found not guilty on the grounds of mental illness.