Fish are brought in by an importer who has to put them through a 4 week quarantine, and when a shipment of fish comes into the QT the mpi vet checks the importer as they unpack the boxes, ticking off each fish you pull out against a list of what you are saying is comeing in, what is on the invoice from where ever is suplying them.
Theoretically they should know what the fish look like and be able to tell when the wrong fish comes in. In practice they don't, especially with fish that are hard to tell like the different palmas's.
So at the end of the day it is mostly relying on the person shipping fish, and the importer to be honest about what fish are comeing in, but often we get species that are not allowed here by mistake, or deliberate.