This might be a bit late but for those who are reading this in the future...
Wainuiomata beach can be on and off but least to say you will come home with some sort of fish.
There are 3 main spots you can fish in this area and they all produce fish. When you arrive you will come across one of the following areas looking south.
Right River Mouth:
You will catch mainly kawhai and red cod here but have occasionally picked up blue cod here. If your in to kawhai bring burley with you and drop it in the river and let it out to sea, you will sure to catch them here.
If you have a fresh water license, this area holds a lot of good pools that are full of sea run trout too
Middle Beach (by the big rock):
You can catch anything from rigs (lemon fish/shark), Moki, Kawhai, Red Cod, Blue Cod. When you are fishing this area, be sure to bring crab or mussels. Fish as close to the rock as you can.
Left River Mouth:
This is mainly another Kawhai area as it is obviously near a river mouth. Although you can also catch Moki here too as it is quite rocky. Once again, burley up in the river and use the current to push it out to see. This area is also quite snaggy at times to be prepared to lose gear although you will also catch fish in this area.
One thing to note here is when it is blowing 20 knt plus winds, DO NOT GO HERE unless you and your gear to be sandblasted. This is an area that is prone to strong winds and gusts. Another time not to come here is when it is blowing southerlies or easterlies, you will see 3-4m swells here during those times.
Tight lines!