How can an aircraft get back to where it started from ready to operate the next days service if the flight is longer than 12 hours and its a daily service, which is what is required to only have two aircraft on the route, and you also need to factor in the BNE LAX service supplies an aircraft for the New York Leg of QF107/108. You will find in any given week due to maintinence etc there is at least three aircraft operating an pair of flight numbers to LAX or DFW:
20/12/2012 QF7 Sydney Dallas-Fort Worth VH-OEJ B747-400 58J/36W/270Y
21/12/2012 QF7 Sydney Dallas-Fort Worth VH-OEG B747-400 58J/36W/270Y
22/12/2012 QF7 Sydney Dallas-Fort Worth VH-OEJ B747-400 58J/36W/270Y
23/12/2012 QF7 Sydney Dallas-Fort Worth VH-OEG B747-400 58J/36W/270Y
25/12/2012 QF7 Sydney Dallas-Fort Worth VH-OEG B747-400 58J/36W/270Y
26/12/2012 QF7 Sydney Dallas-Fort Worth VH-OEH B747-400 58J/36W/270Y
27/12/2012 QF7 Sydney Dallas-Fort Worth VH-OEG B747-400 58J/36W/270Y
If you have a look at any 744 in the fleet using jet tracker on Qanats source you will often see in a given month they have 4-7 days in Sydney, which is why all up three aircraft are required.
To make this easier to understand, I will use times in SYD time for everything below. Lets use 2 aircraft, VH-OEJ and VH-OEG. Hypothetical situation (sorted per round trip to make it easier to read):
QF7 VH-OEJ Dep SYD 21/12/2012 3:40pm Arr DFW 22/12/2012 6:45am
QF8 VH-OEJ Dep DFW 22/12/2012 3:00pm Arr BNE 23/12/2012 7:15am
QF8 VH-OEJ Dep BNE 23/12/2012 8:45am Arr SYD 23/12/2012 10:15am -> becomes the QF7 on 23/12/2012
QF7 VH-OEG Dep SYD 22/12/2012 3:40pm Arr DFW 23/12/2012 6:45am
QF8 VH-OEG Dep DFW 23/12/2012 3:00pm Arr BNE 24/12/2012 7:15am
QF8 VH-OEG Dep BNE 24/12/2012 8:45am Arr SYD 24/12/2012 10:15am -> becomes the QF7 on 24/12/2012
QF7 VH-OEJ Dep SYD 23/12/2012 3:40pm Arr DFW 24/12/2012 6:45am
QF8 VH-OEJ Dep DFW 24/12/2012 3:00pm Arr BNE 25/12/2012 7:15am
QF8 VH-OEJ Dep BNE 25/12/2012 8:45am Arr SYD 25/12/2012 10:15am -> becomes the QF7 on 25/12/2012
QF7 VH-OEG Dep SYD 24/12/2012 3:40pm Arr DFW 25/12/2012 6:45am
QF8 VH-OEG Dep DFW 25/12/2012 3:00pm Arr BNE 26/12/2012 7:15am
QF8 VH-OEG Dep BNE 26/12/2012 8:45am Arr SYD 26/12/2012 10:15am -> becomes the QF7 on 26/12/2012
... etc. etc. This allows a whole 5 hours in SYD slack time.
BNE-LAX is practically the same, as is SYD-x/LAX-JFK (both 744's, sometimes 744ER). LAX-JFK return has no bearing whatsoever on total round trip time, as otherwise the aircraft would simply sit in LAX all day.
The services that do require 3 (sort of) are runs like SYD-x/SIN-LHR and MEL-x/SIN-LHR but QF get away with this by alternating the A380s between LHR and LAX at SYD and MEL (more so at MEL than SYD), eg. inbound from LAX becomes outbound to LHR and vice versa. For the sake of round numbers this means the 2 LHR routes require 3 A380s, and SYD/MEL-LAX require 2, giving a total of 10.
At the moment the SYD-HKG run (1 aircraft) is done 3-4 times weekly, but this could potentially go to a daily service once all the refurbs are done as the 12th aircraft can then act as the spare.