Thanks heaps, David!
So a sudden tail wind seems to have been the culprit - my initial thought was that something had fallen from our old 734 as I couldn't see anything else in the vicinity of 34R and taxiways.
For those interested, I've created a webtrak compilation from QF118's go-around decision point at around 500 ft through to its eventual landing around 10 minutes later. The tags show the relative positions of QF803 and QF118. I was sitting in seat 14F on QF803 - just behind the right wing exit rows - and with a line of sight to QF118 as we banked during the tight right turn off 34R:
http://www.argystar.com/pics/QF118%2...-%20074845.pdf
This may not be that uncommon but it was certainly a novel experience for me from that vantage point!