CAO 95.10 states:
5. Flight Conditions
5.1 (a) the aeroplane may be flown 5 000 feet above mean sea level or higher:
(i) only if it is flying over an area of land, or water, the condition, and
location, of which is such that, during the flight, the aeroplane would
be unable to land with a reasonable expectation of avoiding injury to
persons on board the aeroplane; and
(ii) only if it is equipped with a radiocommunication system;
Note When flying at, or above, 5 000 feet, pilots are expected to make radio broadcasts as set out in AIP.
Had a recent chat about this with a few people as we used to reguarly operate at altitudes up to 9500' AMSL in the Sportstar (a requirement depending on which direction you head from Canberra!). The consensus of the group was that most of us were operating aircraft fitted with relevant comms & transponder equipment cruising at similar if not higher TAS's (100kts +) than many VH registered light aircraft...so there probably an element of 'old rules'...or maybe not 'old rules', just not necessarily appropriate considering the capabilities of many of the newer aircraft available in the category these days.
Ultimately the responsibility for safe operation of the aircraft is up to the pilot in command (you) and as per para (i) in the regulations - if you as pilot in command believe it to be "reasonable" to be at an altitude above 5000' AGL for reasons of safety and comply with (ii) then I doubt anyone is going to question it. For me (as taught to me by my instructors) "height = time" which in any emergency you can't have too much of.
My 2.2c worth.
Cheers,
Matt.