Melbourne High School's former business manager took advantage of the school's poor financial oversight to steal more than $432,000, a court has heard.

For at least 10 of the 25 years Frances Walshe worked at Melbourne High, she breached the trust of the school and fellow staff members by siphoning funds to her own bank account.

At a pre-sentence hearing on Thursday, Walshe's lawyer conceded his client — who has pleaded guilty to 10 counts of theft and one of obtaining financial advantage — would be sent to jail for her offending, which he said occurred because of a "compulsion to gamble she couldn't control".

The County Court of Victoria heard Walshe's role as business manager gave her full access to the school's bank accounts, which enabled her to make payments and transfers without others' knowledge or authorisation.

Prosecutor Lauren Gurry said the deception was uncovered when Melbourne High appointed a new headmaster, Tony Mordini, who undertook an investigation into financial irregularities in 2021.

When Dr Mordini and department staff confronted Walshe in a meeting, she admitted the transfers had been going to her.

"Following the meeting, Ms Walshe resigned her position at Melbourne High School and apologised for the 'grief I have caused both you and the school'," Ms Gurry said.

Ms Gurry told the court Walshe's offending began in 2012 when she stole $26,769, sending money from the school to her bank account.

Over the next decade, Walshe made more than 200 other transactions and stole a further $331,411.

The court heard she also inserted her bank account details into invoices, meaning some payments to the school were diverted to her.

Walshe feels ashamed, guilty over offending: lawyer

The court heard Walshe had not paid back any of the money due to her gambling addiction crippling her savings, and she had just $10,000 in superannuation.

"She's lost the trust and support and friendship of many people," defence lawyer Hayden Rattray said.

"The reality is it's unlikely she'll find regular, ongoing full-time work in the future."

Walshe's unchecked power was contrary to the Department of Education and Training finance guidelines, which recommend the principal and a school council member sign off on school bank transfers.

However Mr Rattray said her offending "didn't have the catastrophic consequences that courts often see".

"[Melbourne High] did not suffer significantly," he said.

The court heard the school had declined to provide a victim impact statement.

Mr Rattray said his client, now 65, felt ashamed and guilty, and had been participating in gambling rehabilitation programs since her arrest.

Walshe, who was supported by her three adult children, made no comment as she left the court with solicitor Bernie Balmer.

She will be sentenced next month in the County Court by Judge Peter Lauritsen.