Optus have transparency issues with pre-orders. Been a many posts with people doing so (pre-ordering or ordering and being told stock is available) and then finding there on 'back order' somehow.
I would indeed suggest you just get the phone at Apple themselves. They have a 24 month interest free payment plan and you can then hook the phone up to any other plan that you wish (Optus included). This can save you considerably depending on your data needs. For example a Boost prepaid 12 month SIM will get you 25Gb a month for $20 - For a post paid plan you'd be starting around $60 for that.
Always read the fine print. - That's a given 🐵
From what I can see (and I didn't look too hard) its only $5 a month (over 24 months) but if you pay the amount of early (so perhaps over 12 months) then the $5 fee no longer applies. So $60-$120 in fees over two years. If you take up the $25 a month plan instead of the $60 plan then you're ahead after a couple of months.
Apple not having stock till then might change things for the OP. I'm getting the distinct impression overseas electronic supply chains are not at their usual level these days.