It's a bit of a stretch to claim that the risk-free rate is 3%, when the RBA cash rate is only 1.50%. Yes, you can get higher than 1.50% at call with many banks, but often these accounts have limits on the amount that can earn those high interest rates.
Regardless, the dividend is a corporate finance decision, and results in a bank reducing its assets in order to make the payment to shareholders. If and when they run into financial trouble, they have a greater need to raise capital than if they had reduced or eliminated the dividend. As such, it is of little consideration when deciding if/when/how to short Australian banks.
If the dividend yield is of concern, then a short position in Australian banks could be paired with a long position in other Australian shares, and/or an ETF containing Aussie shares.
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