Hi Louise,
Well you would first have to fly from Manchester to London and then from London to Dubai so it largely depends on how long you have to wait in London for the second flight.
British Airways currently estimate a minimum time of 9 hours 10 minutes and a maximum of 11 hours 30 minutes
for the whole journey. You didn't say which part of Australia you were thinking of (its a big country !) but a direct flight from Dubai to Sydney would take approximately 14-14.5 hours.