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ddorfer86 
The T5-T3 transfer sequence takes about 30-45 minutes but much of the this depends on how busy security is at the T3 flight connections centre.
Disembark and walk from gate to bus stop downstairs: 5 minutes
Wait for bus: 0-5 minutes
Bus journey: 10 minutes
Security: 10-20 minutes
Bear in mind though boarding for your QF flight may begin about 40 minutes before departure, so your connection looks very tight.
On the positive side, BA does provide assistance to passengers on short transfers - i've seen this in action on multiple occasions since this service was introduced earlier this month. You can read more about the new Transfer Support service on the oneworld site, see link below.
http://www.oneworld.com/ow/news/details?objectID=23140