Extracted from statement by Qantas spokesman David Epstein:
..."The aircraft most likely encountered what is known as convective turbulence, which led to it rapidly gaining around 800 feet in altitude before returning to its cruising altitude of 38,000 feet,"...
...Although some media reports had suggested the aircraft was travelling through thunderstorms at the time of Monday's incident, there was no evidence of this....
...There is nothing to link the aircraft to anything untoward...
Unquote
Presumably part of the 'return to the cruising altitude of 38,000 feet' must have been somewhat rapid judging by the many reports from passengers of a 'plunge' and people hitting their heads on the ceiling.
The 'West Australian' reports that two air safety authority investigators have inspected the plane & downloaded Flight recorder data.