There is a huge over-supply of pilots in Australia and it has always been so. So an arbitrary figure of 500 hours command twins was thought of countless years back by operators as a simple means of culling the numbers - like culling the over-supply of kangaroos (well, it is Australia in'it?)
Then some bright spark thought of further culling by asking for 3,4, or even 5 instrument rating renewals. The fact that the applicant could renew his instrument rating each year without having flown one hour in IMC didn't matter.
A friend of mine applied for Virgin Blue last year and didn't get an interview. Why not? Well he only had 5000 hours of F/O time on B747-400's flying around the world, but without 500 hours command on a Seminole or Apache or similar, he was deemed not acceptable to fly as F/O from Melbourne to Sydney and return in gin clear weather. He is now a 747 captain overseas.
A typical Aussie culling is with the RFDS who demand 500 hours instrument time before a pilot "qualifies" for RFDS status. And all the RFDS is is just another GA outfit. The RFDS bleat incessantly that they cannot find enough experienced pilots. There are a thousand experienced pilots that would welcome a job with the RFDS, but the culling madness prevents them from demonstrating their skills.
There are first officers with Dragonair on the A330 with no more than 300 hours total time and have proved first class crew members with exceptional knowledge and piloting skills. So the hours do not necessarily mean competency. 500 hours command twin in GA usally means you must have broken the rules in order to survive without getting the sack for being straight down the line. Yet some operators remain proud of their insistence of 500 twin command before they give you a job.