If you have large amount of AUD dollars, what would suggest for investment? Should I consider some of the wealth management products with the bank in Singapore?

No. just buy the index funds etfs like VOO or CSPX on low cost brokers and do it yourself. read books and pay attention to finances. these financial institutions DO NOT have your interest at heart. their platform and management fees and incompetent advisors will never work out for you.

Holiday and fd

I am holding on to some AUD as well. TBH, I don't have much faith that it will go back to anywhere near 1:1 vs SGD in the short term. In fact, it may continue falling. I intend to use it if i travel for holiday.

I will bite the bullet and exchange if have anything more than $5-10K.

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It depends on the return you aim for and the risk you are going for.