The single clue that could crack the Samantha Murphy case wide open - as expert reveals what could be on her phone

A leading criminologist believes Samantha Murphy's phone could reveal her final movements after the device was found in perfect working condition beside a dam.

Victoria Police search crews last week discovered Ms Murphy's iPhone in the mud on the water's edge at Buninyong, about 19km from her Ballarat home.

She was last seen leaving her Eureka St house in Ballarat about 7am on February 4 to go for a 14km run through the nearby Woowookarung Regional Park.

The device, protected by a teal-covered case, is undamaged and in working condition with investigators hoping it could lead to the discovery of her body.

Dr Xanthe Mallett, a criminologist from the University of Newcastle, said she was 'surprised' to hear the phone was in working condition.

'The police were searching for it obviously for almost four months, something took them to that dam,' she told Seven's Sunrise on Tuesday.

Dr Xanthe Mallett, a criminologist from the University of Newcastle, said she was 'surprised' to hear the phone was in working condition (pictured on Sunrise on Tuesday morning)

Victoria Police search crews last week discovered Ms Murphy's iPhone in the mud on the water's edge at Buninyong, about 19km from her Ballarat home (pictured)

'That hasn't been disclosed what evidence or intelligence that was yet, but yes, this could absolutely offer new clues that will help find Samantha.

'As we know now, it is operational and downloadable.'

Dr Mallett told the program that a phone that had been underwater or in mud for four months would be 'beyond retrievable'.

'So does that indicate that it hasn't been there for the entire four months?' she said.

'That's yet to be seen. But it's that timeline that the police will be working on and it's certainly going to be essential in helping to develop that.'

She said the police would get as much information from the phone as possible.

'I think that there may be potentially more witnesses that may come forward because we now know about that dam, we know the phone is there,' she said.

'So I'm hoping that now that information is public, other people may come forward and offer information as to who may have dumped that phone and when.

'You have to commend the investigators that have worked so hard on this.

'You can see the relief and almost celebration. I know we're not at the end of this investigation yet, but it's a huge development.'

The mother-of-three was last seen leaving her Eureka St house in Ballarat about 7am to go for a 14km run through the nearby Woowookarung Regional Park

Cybersecurity expert Nigel Phair previously said results from the forensic tests could lead to a major breakthrough.

'The physical properties of the phone will obviously be damaged but what's behind it, those ones and zeros of data, will be retrievable,' he said.

'This is the game changer for the investigation.'

The device was found close to where it last made contact with a nearby tower in the Buninyong region before going silent.

The phone is believed to have been submerged in the dam since February 4 - the very day police allege Patrick Orren Stephenson, 22, murdered Ms Murphy while she was out on a Sunday jog.

Daily Mail Australia was told this week Missing Persons Unit detectives had been suspicious of the dam soon after taking charge of the investigation, questioning its owner just weeks later.

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Police would not return until early May when they asked the owner again if he would mind police entering his property.

Three weeks passed before Wednesday's search turned up the phone.

The location of the dam sits just around the corner from the Durham Lead Nature Conservation Reserve, which search crews descended upon on April 12. The dam had been at its lowest since the heat of summer in December.

The owner of the dam, who wished to remain anonymous, suggested police had no specific information the phone had been discarded there.

It comes as Stephenson hired top criminal defence lawyer Paul Galbally from Melbourne-based law firm Galbally O'Bryan.

Mr Galbally represented the late Catholic Cardinal George Pell, who had his conviction for historic child abuse charges overturned by the High Court.

He has previously said he is not uncomfortable acting for individuals accused of the most serious crimes.

'You either have a disposition or a personality that can deal with this work or you don't,' he said in a 2009 interview.

The Galbally O’Bryan website states Mr Galbally has 'run some of the country’s largest and most complex criminal cases'.

'His experience includes representing clients before public inquiries, royal commissions and in defence of Australian Securities and Investments Commission prosecutions,' the website adds.

'Paul is recognised throughout the legal profession for his judgment, sound strategic advice and discretion and is featured in Doyles Guide as one of Australia’s pre-eminent criminal defence lawyers.'

More to come.