Shareholders' equity - Ordinary shares purchased and sold on market (Details)	6 Months Ended
Mar. 31, 2021 $ / shares shares
Ordinary shares
Share-based payments
Net number of ordinary shares purchased/(sold) on market	3,201,544
Employee Share Plan (ESP)
Share-based payments
Average share price, ordinary shares \| $ / shares	$ 19.09
Employee Share Plan (ESP) \| Ordinary shares
Share-based payments
Number of ordinary shares purchased on market	1,178,527
RSP
Share-based payments
Average share price, ordinary shares \| $ / shares	$ 20.74
RSP \| Ordinary shares
Share-based payments
Net number of ordinary shares purchased/(sold) on market	1,890,323
Westpac Performance Plan (WPP) \| Share rights
Share-based payments
Average share price, ordinary shares \| $ / shares	$ 19.07
Westpac Performance Plan (WPP) \| Share rights \| Ordinary shares
Share-based payments
Number of ordinary shares purchased on market	132,694

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The weighted average share price. [Refer: Weighted average [member]]

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Ordinary shares purchased on market for share-based payment arrangements.

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The net number of ordinary shares purchased or sold on market.

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