Take this from a penultimate student doing actuarial studies at UNSW who knows plenty about Macquarie too, it makes no difference where you go. Your double degree will most likely not benefit you at all. I started off in mathematics then changed to economics and then dropped that too, my marks were pretty much HD in all of them but didn't feel like racking up HECs debt on subjects that won't even help in the end. It hasn't stopped me from getting multiple internship offers, they seriously don't care what degree you do as long as it has an actuarial studies somewhere in there. Choose which ever uni is more convenient, I chose UNSW because most of my friends went there and also heard Umacq was full of bogans
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As for career options, you're probably guaranteed A job when you graduate, not necessarily an actuarial one. My role in the Actuarial Society is somewhat like a career adviser so my information is pretty reliable. There's about ~30-40 intern positions available to penultimates every year, and graduate roles ranging anywhere from 60-100 (this is all of Australia). UNSW + Macq pumps out about 100-130ish graduates a year, plus maybe another 30 around the rest of Australia, so it is fairly competitive. But if you go for a generic finance role you'd probably get it since actuarial studies is regarded pretty highly elsewhere. Companies don't really hire based on uni, this year all 4 PwC interns are from Macq, and all 4 EY interns are from UNSW, it doesn't really matter what uni you go to.
Difficulty-wise it is pretty hard, but if you study consistently it isn't impossible to maintain a HD average even into your penultimate year, I know a handful who has it apart from me. Drop out is pretty high, my cohort started with about 200 people, down to maybe ~100 now. People do drop out because they can't get the marks, but a lot also just realises that actuarial isn't for them.
I've seen quite a lot of dodgy to outright wrong info in this thread so just thought I'd clear it up. PM me if you have any more questions.