David,
I think that in most cases common sense would prevail. If treated fairly most officers would most likely give you a warning. If you had purchases mullet fillets for bait, then in most cases I think proof would be in the packaging and receipt. If it were some other fish it would have to have its skin and scales still attached (mullet included) so that the DPI officer could identify it as not being a regulated species..
Whilst I understand the below link is a guide only I reckon if you had a copy of it with you, and your 'bait' was legit then it could be easily explained away.
However, and I stress however, if you follow to the letter of what is stated (by my interpretation) then any 'mullet' fillets less than 40cm in length would be outside what is acceptable.
Recreational fishing rules and regulations for Queensland
A brief guide
March 2009
http://www.dpi.qld.gov.au/documents/Fisheries_RecreationalFishing/RecFishing-RegsGuide.pdf
Skin, fillet and pectoral fin removal
In relation to coral reef fin fish, a recreational fisher:
• must not possess on board a boat a fish other than in any of the following forms—whole, gilled, gutted or filleted
• may possess on board a boat a fillet of a fish other than a blue spotted coral trout (Chinese footballer) as long as the length of the fillet is at least 40 cm, and the skin and scales of the fillet are attached to the fillet
• must not return fish to a boat that have been taken ashore from a boat and filleted, unless the length of the fillet is at least 40 cm and the skin of the fillet is attached to the fillet
• must not possess a dead coral reef fin fish unless a pectoral fin has been removed from the fish
(my interpretation here is that it is legal to use a regulated fish as 'bait' but the pectoral fin must be removed and must be of legal size. It would also be counted towards your bag limit).
or
• must not possess a live coral reef fin fish unless the person or fisher intends to immediately return the fish to the sea or to display the fish in an aquarium.
In relation to fin fish (other than coral reef fin fish) a recreational fisher must not:
• remove the skin from a fish on a boat until the fish is brought to shore
• bring a fish ashore and remove its skin and return the fish to the boat
• divide a fish into portions other than in a way that allows an inspector to easily count the number of fish possessed by the fisher
• possess a dead Spanish mackerel unless a pectoral fin has been removed from the fish.
Note: For information applying to recreational fishers on commercial fishing tours, consult the fisheries legislation at www.legislation.qld.gov.au
Hope this helps
Shane