Thousands of Australians are expected to save $120 million a year as part of a new banking system designed to stop online purchases from dodgy overseas sellers.

From Monday, Westpac says its customers will have their credit and debit card transactions to overseas retailers automatically blocked if a scam is detected.

They will be notified of the intervention via text but can contact the bank if they believe the purchase is legitimate and wish the transaction go ahead.

When the real-time system was trialled last year, customers who received the scam alert did not proceed with the purchase in more than 99 per cent of cases.

The initial trial, which involved select online stores with dubious marketing claims including diet pill retailers, blocked more than 11,000 potential scams for 4000 customers, saving more than $1.2 million.

Westpac’s Chris de Bruin said online shopping scams are on the rise and more overseas retailers are targeting Australians with false advertising.

“Over the past two months, we have stopped 69,000 customers losing more than $6 million through our scam blocks with customers saving an average of $87,” he said in a statement.

“Through expanding this to more merchants, our modelling predicts we will stop an additional 2000 customers each day losing an estimated $120 million annually to dodgy overseas retailers.”

Under the system, merchants being blocked are exclusively registered overseas and PhenQ Reviews most commonly sell diet pills, nutrient supplements, online dating subscriptions, music subscriptions or business support services.

“We want to help put a stop to online scams by blocking retailers who may be misrepresenting what they’re selling or duping Aussies into handing over money for products they’ll never receive,” Westpac’s head of fraud Ben Young said.