15-20 years ago, the rules regarding fuel carriage were changed. Prior to the change, the requirements (e.g 45 minutes FR, 15% VR) were spelt out in AIP. Now, there is just one rule:
Thou shalt not run out of fuel - Reg 234, CAR 1988
What had been
required fuel carriage prior to the rule change became
recommended from then on. This was effected by transferring the various paragraphs out of the AIP and into an advisory document (CAAP 234-1).
So what does the CAAP document say about fixed reserve? In the definitions section, for "fixed fuel reserve", it says:
fixed fuel reserve means an amount of fuel, expressed as a period of time holding at 1 500 feet above an aerodrome at standard atmospheric conditions, that may be used for unplanned manoeuvring in the vicinity of the aerodrome at which it is proposed to land and that would normally be retained in the aircraft until the final landing.
Following the rule change, commercial operators were required to spell out their fuel carriage requirements in their operations manual. This allowed the operators to devise fuel requirements that were appropriate for their operation. So, the definition of "fixed reserve", and whether it is even required, is up to the holder of the AOC (in the case of a commercial operator).
Of course, private operators are also free to devise their own requirements - the CAAP document only makes recommendations.
You can download the CAAP document from here:
http://www.casa.gov.au/download/CAAPs/ops/234_1.pdf