NP from November onwards is good. Best months being late DEC,JAN,FEB.
Usually the river mouths are pretty packed with kahawai fisherman, so it pays to come back from the mouth about 40-80 meters. Depending on the shape of the mouth, thats the best spot anyway, right where you get the big back swirling eddies with all the good wash in them.
The biggest secrete to catching livebaits at a river mouth is to catch them in the lagoon in the river !! All the big mullet sit in the still water out of the current in the river side. Just take a decent container with you and full it up and catch 5 at a time. Kahawai will smash a big yem too, but that is something you just have to put up with, as soon as a king comes along, wack!
Off the beach, 60lb fluro is good, thick enough not to tangle, and light enough not to put fish off in the shallows. I would also look at 15kg mainline because they all tend to cross the river mouth and its hard working bringing them back arcoss on lighter gear as most local wont give a shit and will keep tossing their lures over the top.
They are a fish that can be caught all day. usually the top half of the incomming is best, but they will turn up as they please. Cleaner the water, the better your chances.
Top three spots up my way
Snapper rock has heaps, I got 7 in two days earlier in the year. Most are not very big but it is choice fun, usually upto 15kg. Average size around 6-8kg
Mohaka, this place has quite a few, not as many as snapper rock, but the fish here is much bigger. On the right day you can have the time of your life, my best fish here is over 20kg. That day I had a double hook up and could only land one but they were both huge fish. The average size is around 12kg
Wairoa Bar, now this is a truly bloody awesome place to snag a kingfish, and personally my favourite. There is allot of water that pushes out here and the amount of kahawai life is phenomenal!! I have got kings over 20kg here also. The average size again is around that 12kg mark, but there is some whoopers here.
Shot taken of the Wairoa bar, with a king on top of the wave