Hi guys,
To be honest there is not much happening with bush/charter jobs in Australia at the moment, that is to say there are a few around however they mostly wan't around 800-1000 hrs total with some command hours on the type they are using along side some previous bush time. Then there are the issues with visa's and licence conversions, probably not something you wan't to try and get into with a working holiday visa. to convert your JAA you will need to first pass the CPL air law exam (not too hard with a week or two study). Then pass the irex exam and complete a check ride to validate your IR. There also would be issue's as most larger airports require an ASIC card to be worn at all times, it can take a couple of months to get passed all the red tape and get one issued.
I would suggest you would need 6-8 weeks to get the conversion done plus however long it takes to get the security clearance for the ASIC card. A 12 month working holiday visa allows you to work and/or study for a maximum 3 months, so you may get a couple of weeks in flying if you are lucky and you would have to be very lucky..