If you'd like to have a go at messengering in Sydney the best (almost only) place to start is Mail Call. They are always hiring and people are always leaving, they don't really look after you (hence people leaving), but if your good at looking after yourself you might go alright. I'd say that perhaps 1 in 30 mail call couriers does well enough to consider doing it for more than 12 months and if your good you may end up putting your hand up and getting poached by a better company. Either way its worth it for a bit of fitness and fun. Even if you work for a few months and decide its not for you. Going by your pic you may need to change the set up on your bike, Mail Call only hires people with freewheels, brakes and bells (and reflectors or lights attached to your bike when they inspect it).
If a knife carrying cyclist is stopped by the police, how do you explain carrying a weapon in a public place? But police would expect a cyclist to carry a bicycle lock. Sometimes when cycling alone at night, I place my lock over the handlebars within easy reach. A lock reaches further than a knife and easy to swing. That is, if you have the chance to defend yourself, because the attacker has the advantage of surprise and first move.