I'm thinking if AUD weakens, then that helps GOLD asx fund rise
Another way to think of it is simply to look at the gold price in AUD and ignore the USD aspect of it altogether.
That said, for some practical examples to illustrate the point:
AUD is currently = USD 59c and gold is at AUD 2551.03 or USD 1507.10
If the gold price remained constant in USD but the AUD dropped to USD 50 cents then in AUD the gold price would be $3014.20
If the gold price remained constant in USD but the AUD dropped dramatically to USD 30 cents then in AUD the gold price would then be $5023.67
If the gold price remained constant in USD but the AUD rose to USD 70 cents then in AUD gold would be $2153.00
That said, a complicating factor with using gold as a hedge against currency movement is that the gold price is itself not constant against any currency.
If you specifically wanted a simple way to own USD as a currency hedge then the currency ETF's are one way to do that. USD (US Dollar), POU (British Pound) and EEU (Euro) trade on the ASX the same as any other share or ETF. That isn't the only way, just an easy one assuming you already have a brokerage account with cash in it.
Note that's for information only and is not a specific recommendation to buy or sell anything.