Mandorah Jetty is directly across the bay from darwin. You can get there fairly easily by ferry. This wharf is great for spanish macks, queenies, longtail, trevs and the odd barra under it. Livebait can be caught here and is by far the best bait for the species mentioned. Metals and poppers would go well here too! Stokes Hill wharf near the main part of darwin is good too. Queenies, macks, trevally, barra and the odd tuna here. At night the barra bite comes on!
Not a darwin local but have a few mates up there that fish it a fair bit. Been there a few times too. It may be worth hiring a guide for a day and getting him to show you some productive techniques and locations. Milkfish is another option too! Go like freight rains!
Cheers, Tom