A Perth man has been sentenced to almost four years in jail for stealing money from his late girlfriend's superannuation, by using her phone within hours of her death to access her account.

Travis Paul Barnard was found guilty of one count of using a telecommunications network with intention to commit a serious offence, after the Australian Federal Police charged him.

Kim Heptinstall died after surgery in July 2019, and the charges came after the woman's relatives raised concerns about her financial affairs.

Barnard was found to have used Ms Heptinstall's phone to access her superannuation and bank accounts.

He changed the details of the non-binding beneficiary from her estate into his own name, to gain access to more than $300,000.

He also withdrew cash after transferring money between her bank accounts and using her card.

He attempted to conceal the offences by deleting her banking and superannuation apps, and signing out of her email account before handing over her phone to her family.

Blaming 'a phantom'

Barnard, who was assessed as having borderline personality disorder in 2015, originally denied committing the offences, before placing the blame on mental health issues at trial, claiming it was "the other Travis".

In sentencing, District Court Judge David MacLean rejected that claim as baseless, saying it was an excuse to attribute responsibility "to a phantom".

He gave a damning assessment of Barnard, labelling him a deeply dishonest person, who showed no remorse or any indication of taking responsibility.

Judge MacLean said the offending was a cold-hearted, calculated breach of trust, and accused Barnard of making up stories.

He also said he didn't accept Barnard felt any sense of grief over Ms Heptinstall's death.

He rejected the defence's argument that Barnard had a genuine desire to pursue psychological and psychiatric help, telling him he had no interest in fixing "what makes you tick".

Judge McLean also highlighted moments in the trial where Barnard claimed not to recall basic information about himself, including his employment and financial status.

He said there was a high risk Barnard would dishonestly offend against someone again, and the only appropriate penalty was immediate imprisonment.

The judge sentenced him to three years and 10 months jail, backdated to March 25 this year for time already spent in custody.

He'll have to serve half of that sentence before being eligible for parole.

The maximum penalty for the type of offence is seven years.

Family speaks of 'hell'

Speaking on behalf of the family, Ms Heptinstall's older brother Ben Heptinstall said it was a satisfying outcome.

"It's a relief that it's finally come to light, because we've had to live with the lies for the last five years," Mr Heptinstall said.

"For the judge to finally acknowledge and accept what really happened in those hours and days after my sister's passing is a massive relief for basically what we've been fighting for, for the last five years.

"I'm very satisfied that the truth has come out and that they were all over every aspect of what happened after my sister passed away."

He said the family's ordeal had "been hell".

"After my sister passed away we were organising her funeral, not knowing what was going on behind the scenes, and it wasn't until after the funeral that this all sort of came to light, and we were just trying to grieve the loss of my sister," Mr Heptinstall said.

"Now we had this other obstacle we had to fight for, so it's been tough."