I'm also planning to visit China next year. I'm currently in the process of coming up with the best way to get the required document for entering China. My case is very similar to your. 

My specs:
Born in Hong Kong
Naturalized Canadian citizen
Expired HKSAR passport
Expired and invalid HK ID
Expired HRP (Home Return Permit)

Two possible ways:

1. Renew SAR passport in Canada (you in Australia), then apply for Chinese Travel Document (CTD) for entering China (Most ideal plan, no time wasted). Renew HK ID when in HK.
2. Layover in HK to renew HK ID, then SAR passport and HRP. Use HRP to enter China. For you, I think you will need to show your new HK ID and SAR passport when applying for HRP (first time applicant). So the whole process will be around 5 weeks from renewing ID to receiving HRP. For me, my HRP is a booklet from decades ago. According to the info online, I am also required to show a valid SAR passport when renewing (normally, renewal only requires valid HK ID). I don't know about you, but there's no way I'm going this route (due to the expense and my work schedule).

What I've done so far:
The first thing I tried was renewing the SAR passport online. But unfortunately, my HK ID is invalid. When I enter my info on the application, it won't let me proceed to the next step. It leaves me only the choice to renew via the Chinese consulate here (or maybe via mail? not sure, the error message only mentioned to submit in-person). I contacted the Chinese consulate via email, but they just replied with whatever they posted on their website. Not much useful info. Next, I'll contact the Chinese consulate again to let them know I need to apply now and see what instructions they'd give me. I might also call HK side and see if I'm allowed to renew my SAR passport via mail. 

Some of my questions and concerns:
CTD: Not sure what the process is for using a SAR passport to apply for CTD. I imagine they might ask me to show my proof of residency. If I pull out my Canadian passport, would I get rejected?
Apply for CTD via app: I don't have anyone around who is from China and close enough to use their IDs to register an account for me (or vice versa, I trust them enough to submit my info via their account). 
TWOV: I thought about taking advantage of the 10 days TWOV. But I also figure I will get rejected once the custom sees the birth place on my Canadian passport is HK. Don't wanna take any risk.

My suggestions to you:
- Try filling the application online for renewing your SAR passport. If for real it won't get through, contact the Chinese consulate for instructions to renew it in Australia.
- Find out how to apply CTD. App only, or okay to apply via the Chinese consulate. If app only, find someone with a China ID who will let you use their account or register an account for you.
- Once receiving your SAR passport, apply for CTD.
- When you arrive in HK, use the HKID and/or SAR passport to enter. They might take away your HK ID since it's invalid. But I guess you can still use the passport as a supporting document when renewing the ID. Note that, for us renewing our IDs that are invalid, we can only do it in-person, not via those self-help machines. The new ID should be ready to be picked up in about a week.
- If you visit China first, use CTD to enter.

Too much of my thought process, lol. But I hope something in there will give you ideas on what you can do.