A Perth Mint security guard who stole a $50,000 gold bar by hiding it in his underpants has been sentenced to 15 months' jail.

The District Court was told Matthew Alexander Roussety, 27, had a "brain snap" when he decided to take the one-kilogram bar while he was working at the mint's refinery at Perth Airport in December 2016.

The bar was a reject one, and was going to be melted down, but Roussety decided to steal it by secreting it down the front of his pants.

It set off metal detectors when he was leaving the refinery, but he managed to explain it away by telling security officers the alarm had gone off because of the metal zip on his trousers.

Three months later, after he had left his job, Roussety took the bar to a second-hand gold dealership and after signing a declaration that he had paid for and owned the bar, he was paid $44,500.

Roussety then used some of the money to pay car and credit card debts.

His crime was uncovered when the gold dealership tried to sell the bar back to the Perth Mint, but it was told it already belonged to them.

Suspicion, low staff morale after theft

Matthew Roussety stole a gold bar from the Perth Mint by hiding it down his pants. ( Supplied: Facebook )

Judge Bruce Goetze told Roussety he had been motivated by "greed and self-indulgence" and had acted "impulsively" to relieve himself of financial difficulties.

Judge Goetze said the theft had a negative impact on the Perth Mint, with customers querying its security arrangements and other staff coming under suspicion, causing low morale.

The court heard the theft had also led to the Mint considering a requirement that staff wear metal-free clothing — something that had caused a problem with unions, because it may discriminate against women.

Judge Goetze described the offence as "very serious," telling Roussety he had breached the trust that had been placed in him.

"The very thing you were employed to do, was the very thing you offended against," Judge Goetze said.

Roussety and members of his family in the public gallery cried as he was sentenced to 15 months' jail.

He will have to serve half the term before he can be released.

Roussety has already paid $25,000 to the gold dealership and the court heard he has agreed to pay them $400 per month to pay back the remaining amount.