G'day Wade,
I'm sure there are kahawai and trevally etc up in that estuary, maybe kingies at certain times when the tide is high in summer.
If you are serious about some bigger fish, I recommend the short drive back over the hill to Mangonui Wharf.
I took my 3-year-old there at Easter and we got some nice keeper snapper, and kahawai. It's famed for kingies, probably more of a summer fishery though.
Take some sabiki rigs and try for some livies. Good social fishing, the locals are welcoming if you take it slowly and introduce yourself.
You mention mullet in your cast net. You may be fully aware of the regulations and going after yellow eyed mullet (sprats etc), but in case you don't know, it's illegal to catch grey mullet using a typical narrow mesh (e.g. 25mm) cast net. The minimum mesh size for grey mullet is much wider, like that found in a typical flounder net.
I'm not trying to be a pest, just letting you know. Check the regulations. My understanding is the only species that can be taken with 25mm mesh are-yellow eyed mullet (herring), piper and pilchard. Any other species that are caught need to be immediately released.