But Crown prosecutor Mark Tedeschi, QC, said an even graver error came the next day when Mr Xie found himself back at the crime before he disposed of the murder weapon.

Mr Xie has pleaded not guilty to the 2009 killing of his brother-in-law, Min Lin; Mr Lin's wife, Lily; their sons Henry and Terry; and Lily's sister, Irene, at their North Epping home. Mr Xie was allegedly motivated by greed, a sense of family shame and a sexual interest in a young woman.

Each victim was found bashed in their bedroom with wounds to the temples, while four of the five had also been asphyxiated.

"The killer had a profound knowledge of the layout of the house and where everyone slept," Mr Tedeschi said in closing arguments to a trial that began in February.

Mr Tedeschi said Mr Xie's principal mistake on the night of the murders was to use a key; Mr Xie, his wife and her parents were the only people outside the Lin household who had keys to house.

"Anybody who enters the house [with a key] shows an incredible degree of arrogance and self-assuredness, and a completely unrealistic belief that they are immune to detection," Mr Tedeschi told the jury.

But the Crown alleged Mr Xie committed a more "egregious mistake" the next day.

The accused allegedly thought the staff or customers of the Lin family's newsagent would find the bodies. But it was the accused's wife, Kathy Lin, who first decided they should visit the family after receiving calls saying Mr Lin could not be reached.

"The accused realised, with some dismay, that he and Kathy Lin were going to be the ones to discover the bodies," Mr Tedeschi told the jury.

The couple arrived at the scene where Kathy Lin discovered "one of the most terrible scenes you could imagine". Later, when she was on the phone to the police, Mr Xie said he had to go pick up her grandparents, refusing her demands that he wait with her.

"Why was it that he had to leave when Kathy was crying, begging him to say? The reason was that he had to dispose of the hammer and he could not risk the possibility of waiting for the police to come. He had to dispose of it first. He disposed of it on his way to the grandparents' house."

Earlier in the day, Mr Tedeschi suggested the disfigurement of the faces of Mr Lin and his wife showed they were the primary targets of the attack.

"There is a good possibility, even a probability, that Irene and the two boys were collateral victims, and the real, primary victims were Min and Lily, because their attack shows intense bitterness and hatred."

The Crown's closing arguments continue. Mr Xie's defence barrister, Graham Turnbull, SC, is expected to sum up next week.