Vortex..What',
It's been a while, but as I remember it you obviously require a CPL if you are to work for a mustering company. There are plenty of pilots out there operating privately with a vested interest in the property they are working on.
You will also require a mustering ticket. I think it's a rating, not an endorsement. the rating is 10 hours with a suitably qualified mustering pilot. You will also need a low level endorsment but I can't remember the numbers etc for that one. Once you have those things your right to go. One thing to remember is that the rating has a currency requirement as well, I think it's 50 hours in a 365 day period but I can't be sure.
Different companies have their own in house requirements that you will need to satisfy as well.
Hope this helps, if not, all the information is in the appropriate books. (ie: CAO's)
Cheers mate