Chinese national Cia Liao allegedly attacked four-year-old Alistair Mach with a pair of garden shears and lay in wait for Mai Mach, 60, to return home from work, Detective Leading Senior Constable Tony Harwood told an out-of-sessions court hearing.

Ms Liao, 45, has been charged with two counts of murder, one count of causing intentional injury and one count of assault and unlawful imprisonment.

She appeared in the St Kilda Road police complex on Wednesday evening and was remanded in custody.

Detective Harwood told the hearing the accused was in Australia on a tourist visa and had been in a relationship with Mrs Mach's husband, Brian, for three years, when his marriage had broken down. The 61-year-old had recently ended his relationship with Ms Liao.

Alistair Mach and his grandmother were found dead in the Varsity Court, Albanvale, home Mrs Mach shared with her husband. Mr Mach was found bound and gagged inside the home.

The hearing was told that Ms Liao turned up at the Albanvale property at about 6am on Tuesday, when an argument broke out between Mr Mach and the accused.

Detective Harwood said Ms Liao then assaulted Mr Mach and bound and gagged him, before entering the bedroom where Alistair, who was being babysat by his grandparents while his parents were working, was laying in bed.

Detective Harwood said Ms Liao attacked the boy with a pair of garden shears. An autopsy revealed he had 18 stab and slash wounds, some of which were defensive.

Ms Liao then waited at the home for 10 hours for Mrs Mach to return home, Detective Harwood said. When she was approaching the house's back door, Ms Liao allegedly attacked Mrs Mach with the same garden shears, inflicting 23 wounds.

A neighbour allegedly witnessed the attack and called police, Detective Harwood said. Ms Liao was arrested at the scene.

Mr Mach was taken to the Royal Melbourne Hospital with minor injuries but was heavily sedated.

Alistair's mother, Amy Mach, is the daughter of Mrs and Mr Mach. Alistair is the only child of Ms Mach and her husband Andrew Kwong.

The accused was asked whether she was taking any medication, to which she said she was taking high blood pressure medication. When asked if she was on anything else, she paused and later said "no comment".

Ms Liao otherwise stayed quiet and was assisted by an interpreter through her short appearance. Detective Harwood said Ms Liao stayed silent during her police interview.

She was remanded in custody and will appear in the Melbourne Magistrates Court on Thursday morning.