The move is intended to cut costs and take advantage of cheaper labour and would affect employees in customer service, direct sales and lending services.
Last year, the bank, which is the largest in the country, shifted 175 jobs in administration to Accenture and offshore services provider Genpact in India.
According to Australia's Finance Sector Union, the exact amount of jobs to be moved in the latest round of outsourcing is undecided, pending an enquiry into efficiency to be conducted by Genpact.
However, sources in the bank have insisted that the proposed outsourcing was by no means set in stone.
NAB spokesman Geoff Lynch said: "Because we're only starting our review we have no preconceptions about what the outcome might be.
"The outcome of a review could be anything ... including offshoring, including maintaining the status quo, including automation. There's no preconceptions," he added.
NAB recently cut its staff by 4.6 per cent as part of cost-cutting measures that has seen its operating expenses fall by 4.4 per cent to just short of $6 million last year.
The bank's profits had taken a downturn in the wake of a scandal involving currency trading in 2004.