There aren't fixed steps in the cash/avios conversion prices. The balance is on a slider and changes depending on exactly how many avios you use. So your question is somewhat open ended.

And there are also 2 ways of doing this. Firstly, reducing a cash fare with avios. 45k avios will save you a little over GBP300 based on a one way cash ticket LHR-SYD that I just looked at.

The second way is reducing the amount of avios on a redemption ticket with cash, up to 50% of the total avios. This is better value than the above example, but again depends on exactly how many avios you use. In my opinion (and based on a mixed class redemption I just looked at), you may find it better value to buy the missing avios than pay part cash, particularly if BA are having one of their fairly regular bonus avios sales and you transfer them over.

Your biggest problem is going to be finding normal rate redemption availability into Sydney. I didn’t see any when I looked just now at a few low season dates, but other cities may be more available. You could research redemption availability via a partner site such as perhaps BA or AA.