When buying lures for marlin get the best you can. PAKULA,TOPGUN,MAGOO,HOLLOWPOINT AND MARLIN MAGIC are all good places to start. 6-10 inch will suit most of what sydney will throw at you. A good marlin lure will set you back around $60-100 rigged so a spread can get a little pricey but there is not much point burning fuel at $1.40L towing around ordinary lures. Keep your hooks razor sharp,check and re-check drags,be confident with your knots and hope you don't have any chinks in your armour because if you do a marlin will generally find it.
Best of luck Shane.