They are buying them back on the market so provided there is...

They are buying them back on the market so provided there is trading volume it's basically compulsory. They aren't buying back your shares unless you sell into the market. Sometimes companies will only buy back shares if they fall below a certain amount, typically fair value as deemed by management, but in this case ANZ has not disclosed any particular target level for the buy back so it's probably just at market prices. We can get a better feel for this once they start to buy shares back. Theoretically a buy-back announcement should drive the share price higher as it reduces the amount of shares outstanding which has the opposite effect to dilution. Earnings per share should go up, as well as dividends per share and all other associated metrics such as the % you hold in the company. This is not guaranteed though and can be offset by other factors (e.g. worsening economic outlook). Over the longer term if the company is in the same or better financial health the shares should end up being worth more.