Westpac has guaranteed not to send any jobs overseas for at least one year.
The bank has agreed not to engage in the practice known as "offshoring" for at least 12 months, as part of a new collective agreement affecting 1,800 unionised workers.
The bank has also promised to try to keep its local workforce of 5,500 at the current size for the year.
The banking union, Finsec, is hailing the deal as a major victory.
Finsec national organiser Michael Wood says the ANZ National Bank sent several hundred back office jobs overseas last year that affected a lot of union members.
He says the union has been pushing for promises of job security to try to ensure offshoring does not become an industry practice.
Westpac says the deal is mutually beneficial and offers its staff as much security as possible in the current economic environment.