I would highly recommend you read up a bit more on inland and outland. I'm no expert, no immigration lawyer or consultant, but I think you may find, if you read more on here, that applying inland and applying for an OWP (open work permit) makes no real sense compared to applying outland (since your VO will be Sydney). You can most likely be totally done with the process of an outland application well before you would have received stage one approval and an OWP through the inland process.
What's important in regards to establishing a common-law partnership is that you can prove 12 months of continuous cohabitation (that first 12 months is critical, and that is where long gaps of a few weeks or more may cause CIC to say that common-law status wasn't established). After you have established yourselves as common-law partners by living together for that 12 months (and being able to provide documentation that proves to CIC that you were living together), then you have a bit more leeway. Let's say you live together for 12 continuous months, no long breaks in that time, and you've got that documented (shared lease, etc.). After that, if you spend a month or two apart for employment reasons, or to visit family, that does not end the common-law status. However, the more time you do spend apart, the more carefully you'll need to document and "prove" your relationship to CIC. If you lived together for a year, then apart for a year, it's going to take more to prove to CIC that you're still in a real and on-going relationship (proof of daily communication, perhaps bank transfers to help each other with expenses, etc.).
If you do get married, you still have to "prove" your relationship is real and continuing, but the burden of documenting that 12 months of cohabitation is lifted. There's no minimum time of cohabitation for married couples.