Disturbing Google search made by Kallista Mutten after she'd slept with her daughter Charlise's alleged killer Justin Stein

The mother of murdered schoolgirl Charlise Mutten had sex in a park with the man accused of killing her before googling symptoms of a sexually transmitted disease, a court heard.

Justin Laurens Stein, 33, has pleaded not guilty to murdering the nine-year-old on or around January 12, 2022 in the Blue Mountains. 

Charlise's body was found by detectives on January 18, 2022, in a barrel dumped near the Colo River, in the Blue Mountains, west of Sydney.

Stein had spent the night of January 11 alone with Charlise at a property in Mount Wilson, while the girl's mother, Kallista Mutten, was at a caravan at the Riverview Ski Park about 90 minutes away, the NSW Supreme Court was told.

Stein allegedly told Ms Mutten that Charlise was sick and that he had left her with a real estate agent who had come to do a valuation on the property.

In the hours after the schoolgirl was allegedly shot, Stein and Ms Mutten drove to Surry Hills near Sydney's city centre to buy methamphetamine. 

They then went to nearby Centennial Park where Ms Mutten told police they had sex.

But afterwards, Ms Mutten grew suspicious and searched online for 'blood coming from penis after sex' and 'blood coming out of penis after ejaculation'.

She then accused Stein of cheating on her and contracting the sexually transmitted disease gonorrhoea.  

Police alleged Ms Mutten messaged Stein saying, 'you're a liar, cheat and now I've lost my daughter'.

When the couple arrived back at Mount Wilson, Ms Mutten called Lithgow hospital and Blue Mountains hospital having also made online searches for children's emergency rooms in the area.

Ms Mutten left the Mount Wilson property the next day in Stein's red Holden Colorado ute.

'If you don't bring it back I'm going to hurt you as well as everyone else,' Mr Stein said in one of the messages.

'I've got me f***ing guns and I'm going to kill you.'

Stein claims Ms Mutten shot her own daughter. 

His lawyer told the court earlier this week he had initially lied to police to cover for Ms Mutten because the pair were in a relationship.

Stein spent more than two hours being interviewed at Penrith Police Station on January 14, 2022, two days after prosecutors allege he shot the girl twice and put her body in a barrel before rolling it down an embankment near the Colo River.

The alleged killer, who pleaded not guilty to murder but admits disposing of Charlise's body, broke down in tears during the police interview.

He told officers Charlise was his 'little girl' and that she 'means the world' to him.

Stein told police he had left Charlise with a real estate agent on January 12, who had offered to look after the nine-year-old because she woke feeling unwell.

It was after this that Stein and Ms Mutten travelled to Sydney to buy drugs and have sex in a park.

When they returned shortly before 9pm, the house was locked and Charlise was nowhere to be found.

Stein said he had 'no reason to lie' about the real estate agent story, even though he failed to take her name or number, and told police he would say in a 'heartbeat' if anything bad had happened to the schoolgirl.

'That kid really does mean the world to me you know what I mean, I don't have much experience but this was my chance,' he said through tears.

'She was going to be my little girl.'

The officers question why after learning this information Stein didn't seem to take any steps to find out who the woman was by looking into what real estate agency she worked for. 

'There's nothing I can say, I know how s*** looks; it looks bad,' Stein told the officers. 

A number of times during the interview, Stein became emotional, explaining he is under immense stress and had not slept at all the night before.

Stein told police he spent that time travelling to locations across Sydney, including Rose Bay and Drummoyne, to ask contacts from his criminal past if they knew what happened to Charlise.

Prosecutors allege Stein was in fact looking for a suitable location to dump Charlise's body.

When asked who those people were, Stein was unspecific, adding while he didn't have ongoing conflicts, there was 'bad blood' between him and a co-offender in a cocaine importation scheme over which he spent five years in jail.

Stein had also reported his car stolen the day after he said Charlise went missing, with police officers attending the property in relation to that report.

In the video, the officers questioned why Stein didn't tell police at that time Charlise was missing.

'There was police at the house yesterday and there was a report made about a car being stolen but not about a nine-year old being missing,' one of the officers said.

'Can you appreciate where we're coming from?'

The questioning then turned to whether Stein or Ms Mutten had anything to do with what happened to Charlise, either accidentally or deliberately.

The officers suggested Charlise may have been accidentally run over, or taken medication from which she didn't wake up.

'Have you been involved in the death of Charlise?' one of the officers asked.

'No, no,' Stein replied.

When asked if Ms Mutten may have done something to Charlise without his knowledge, Stein said with her history of drug use and mental health issues it 'could be possible'.

'Honestly in my opinion, I don't think her kids mean as much as she says they does (sic).'

The trial continues on Friday.