The short answer is 'Yes, but at an additional cost'

See rule 4H

You have 5 flight segments in the South West Pacific. You are allowed 4 free flight segments per continent, and can purchase up to two additional flight segments percontinent at the rates shown in the rules.

Since your 5th segment is MEL-SYD, and the cost of purchasing an additional economy segment is USD150 (USD400 for business) - it will be far cheaper for you to not buy this, and instead make you own way MEL-SYD on a separate ticket.

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An 'off the wall' idea if you are serious about maximising miles:
Shell out an extra ZAR4,500 and purchase the 4-continent circle trip; start it with CPT-LHR-MEL...