I trend towards lighter lures but have caught heaps on heavier ones. Best to use a lure weight which is a good match for your rod. Could also try Flys because the Kahawai prefer them at times, this thread shows a easy way to cast them out.
Another trick is to tie on a sabiki rig between the mainline and lure, can be highly effective at times but can also result in impressive tangles.
At first Kahawai seem to be easy fish to target which would eat almost anything, this is true. But the more I learn about their patterns they more confused I have become. I personally now beleive that the Kahawai do not care much for the tides but their habits are more based around salinity conditions. When the rivers are in flood they hang far offshore during all tides, when the water clears with moderate flows they start to push in with the incoming tide and leave with the start of the outgoing.. When river flows are extremely slow, they seem to spend most of their time in the river only returning to the mouth over low. But, Kahawai do what they want to do when they want to do it so nothing is certain.