Have you tried your local tackle shop ?
GPS marks are usually only a starter point anyway, the best is to find your own spots, cause fish there one day, gone the next.
Log onto www.fishfinderbooks.com , they have a good book called North Australian Fish Finder. about $30 delivered to your door / PO box etc.
Definately get a good marine chart.
also look up www.tsvestuarycharters.com.au
also could contact Steve Jeston at
hyperspace@bigpond.com for some info.
also try www.fishingcairns.com.au
That's about infoed me out. good luck