On 11 August 2007, a Qantas Boeing 737-400 on a scheduled passenger service from Perth, WA to Sydney, NSW was about three quarters of the way there when the master caution light illuminated associated with low output pressure of both main tank fuel pumps. The flight crew then observed that the centre tank fuel pump switches on the forward overhead panel were selected to the OFF position and he immediately selected them to the ON position. The flight was completed without further event.