Westpac Banking, Australia's second-biggest lender by market value, said a two-day outage of online banking is still affecting some customers as the bank works to restore full service.

A small number of customers may still be affected during this process, Supreet Thomas, a spokeswoman for the Sydney-based bank, said in an e-mailed statement without further detail. About 3.5 million of its 12 million customers use mobile and online banking, it said in a filing in May.

Internet-based and mobile banking is the fastest growing mode of transactions for Australian banks. Westpac's Australian Financial Services unit expects the number of online and mobile payments to rise to more than 83 million a month in 2017 from 43 million last year, it said in a filing in March.

"We take systems reliability extremely seriously and are very disappointed by the inconvenience to our customers and will undertake a thorough review," Harry Wendt, head of Westpac's digital financial services, said in today's e-mailed statement.

Westpac spent A$160 million ($143 million) in the six months to March 31 on technology and information services, according to a stock exchange filing.