Pay negotiations for one of the country's largest private sector collective agreements begin tomorrow, with Westpac workers seeking to 'inflation-proof' their pay in the face of significant cost of living increases.

Finsec Campaigns Director, Andrew Campbell, who is a past president of the Otago University Students Association, says workers want their agreement to at least match the predicted inflation figure of 5.9%

Campbell says the Australian-based bank made $125m in the first six months of this year, and their pay claim will only cost around 2.5% of that profit.

He says a pay increase now, and an inflation adjustment next year, is an accurate and fair way to make sure worker's pay doesn't go backwards.