Measuring finfish
Fin fish are measured from the tip of their snout to the end of their tail (see illustration). If you unintentionally take a fish (including fin fish or crustaceans) that is not the legal size or is unwanted, return it to the water immediately, taking as much care as possible to avoid causing injury to the fish.
Measuring crab
For guidelines on how to measure crabs see Crabs and lobsters.
Measuring device. Fisheries Queensland recommends using a sturdy ruler or coil tape measure to determine the length of catch. Adhesive or stick-on devices can shrink when exposed to weather and be unreliable
http://www.daff.qld.gov.au/28_3011.htm