Simple answer is one flight number, one fuel surcharge. Two flight numbers, two fuel surcharges.
So from Brisbane, for return fares, fuel surcharges are
BNE-MEL-LHR = $26+ $75 + $75 + $26 = $202
BNE-SIN-LHR = $75 + $75 + $75 + $75 = $300
MEL-LHR-MEL (via SIN of course) = $75 +$75 = $150!