The best way is to read the breaking waves and save a route on your plotter. The yellows have been there for a while and I am not sure if they have been adjusted fo this year.
The wave chart time tables will give a good indication of depth of water, available from Christchurch tackle shop.
I time my approach at no less than 1mtr on the chart, this is not an exact calculation and you should approach at the time your confident with.
On Neap tides you will have more time fishing and the chart barely drops to less than 1 MTR. Springs are a whole different matter.
Take this Sunday for instance, the water on the chart drops to approx 600mm at low tide, so 400mm lower than a neap. Thats when the entrance bares it's teeth and depending on the weather you don't want to be their when the ebb is moving very fast and dropping quickly, then 1hr after high might be good advice.
The other conditions to take into account are the direction of the wind, SE is probably the worst, Northerlies being the best. High air pressure will allow the water to drop lower so beware of that.