Savigny and a friend saw it and took it.
Later, they messaged the victim: "you give me money I will give you your phone back" and "don't f... with me lady or I'll ruin your life".
Over the next week Savigny and his female associate discovered the phone held details of all the woman's accounts and passwords.
They went to work attempting to bleed her dry.
"It was a determined and sophisticated plan in which you played a significant part," Judge Emma Smith said.
A KiwiSaver account was set up and Savigny and his friend transferred $105,000 into it. They also changed payee details on 15 transactions, meaning funds were diverted to Savigny's personal bank account. Over the week, $50,000 was bound for his bank.
The victim worked tirelessly with financial institutions and police to have transactions reversed until eventually, Savigny was left with only $300.
McCaskill said his client knew there was something fraudulent going on but he was unaware of the details.
Savigny had stopped taking prescribed medication at the time and was also using methamphetamine, the court heard.
Judge Smith said such circumstances led to the defendant undergoing a manic phase.
He was in the middle of that psychological episode in April when he went to his parents' home, demanding money.
A heated argument developed over Savigny's lifestyle and he punched his father in the head three times.
McCaskill stressed the defendant had now cut ties with the criminal community and ceased using illicit drugs.
The victim of the card fraud said she wanted a sentence imposed that helped Savigny live a healthier, more positive life.
"We all depend on each other's honesty and kindness and integrity to make our community co-operative," she said.
Savigny was convicted of accessing a computer system for a dishonest purpose and assault in a family relationship.
He was sentenced to 12 months' intensive supervision, three months' community detention, 80 hours' community work and ordered to repay the $300.
