There aren’t many flights from Sydney to Iran. The only 2 one-stop option is with Qatar Airways (Sydney - Doha - Tehran) or Emirates which is via Dubai.
You don’t have to worry about getting the transfer rate wrong because it’s 1:1 as long as you transfer before the devaluation in april.
You can transfer your AMEX points to Asia Miles to redeem on Qatar Airways from Sydney to Doha to Tehran/Shiraz or other Iranian cities.
For less convenient 2-stop options, there are various star alliance carriers as well including thai airways and turkish, singapore airlines, etc. There are 2 main routes you could take - Sydney to Singapore to Istanbul to Tehran, Sydney to Bangkok to Istanbul to Tehran. Obviously, this isn’t the best option but is viable. For this you would transfer your points to Singapore Krisflyer for the best redemption rate.
Since, Iran isn’t a particularly popular country to visit with limited flights that doesn’t involve significant backtracking from aus, it is a bit of a chanllenge to get there on points. One tip is to find possible flights you can buy with cash and see if you can redeem points for them.