Hi, I'm planning a trip to Aus and I need advice about: Payment (which CC to use) for flight/hotel,Telco services (roaming/AU local data sim),Local payment methods (cash/ewallets/youtrip/trust bank)

Youtrip and trust are great choices. If you wish to get some credit cards rewards, you can also try out AMAZE.

Australia accepts credit card payments so Youtrip or Trust are great for that.

If you find that the AUD relative to SGD is about to strengthen, you could start buying some AUD and store that in the Youtrip wallet. Of course, take into consideration whether the potential FX gain outweighs and potential interest earned on SGD.

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SIM card there is cheap. You can get it the moment you land at the airport

Youtrip

Can consider trust card for your overseas credit card spend since there's no foreign transaction fees. However, if you value card rewards from other banks, then you may need to assess if the foreign transaction fee is worth the card rewards.