Letter box post depth needed

I have letter box which will be put on a post which I have. The post is treated pine, should last for a while. I cannot find definitive information about the depth of the post. The only indication is that a third should be in the ground, so the 1000 mm above ground means that 500 mm should be below ground. Advice on this would be valued. TIA.

 

That maths sounds good. I wouldnt think a TP pole is best practice esp since it will rot and are thick and this makes the depth issue less stable. They are never quite straight either. The box would also be screwed (?) into the end grain which isnt great. Bunnings sell a range of metal letterbox posts to which the box is usually screwed / bolted etc and can be colour matched. From $15

Letterbox Posts available from Bunnings Warehouse

 

Thanks. I was going to use four pretty hefty brackets to secure the box to the post. A metal post would cost a bit more but would last longer, so I'll look at Bunnings.

 

I would check with council as there are legal requirements for how to set it up.