What is the diameter of a galvanize steel Post, if I use a 200mm by 75mm bearer?

To determine the diameter of the galvanized steel post needed for a 200mm by 75mm bearer, you would need to calculate the diagonal distance of the bearer. This can be done using the Pythagorean theorem:

Diagonal distance = √(200^2 + 75^2)
Diagonal distance = √(40000 + 5625)
Diagonal distance = √45625
Diagonal distance ≈ 213.6 mm

Therefore, you would need a galvanized steel post with a diameter of at least 213.6 mm to support a 200mm by 75mm bearer.

The information provided does not specify the diameter of the galvanized steel post. I cannot determine the diameter of the post from the provided information.