Transaction numbers are increasing 20 per cent a year at Westpac, as customers check their balances more frequently and make payments around the clock.
CIO reported that the early hours of Sunday are the only real quiet time in Westpac's weekly cycle.
Mainframe chief engineer Glenn Bowden told the news service that the bank effectively operated an internal cloud on its network of 10 mainframes (eight in Australia and two in New Zealand).
Bowden said: "I don't think in the foreseeable future that we would look at external cloud for mainframes. It's just not viable because of data and security issues."