The web page has already been updated:
+ From 1 February 2011, to access Qantas Domestic or International lounges, Platinum members must be travelling the same day on a flight that shows a Qantas (QF), a Jetstar Airline (Jetstar (JQ), Jetstar Asia (3K) or Valuair (VF)) or a oneworldŽ flight number on your ticket. Other lounge access conditions apply. See Qantas Club Terms and Conditions.
I Guess "travelling the same day" can easily mean departing
OR arriving that day.
Still, it's somewhat ambiguous.