What size box steel posts?
It would seem to me that the 100mm square tube posts would be a bit chincy for holding the hinges for gate of that weight. If it was me I would probably be looking for something a bit beefier like a 5"x5" or a 6"x6" steel tube with a wall thickness of at least 0.25". 

Gates will place a tremendous torque on the posts and I also have my doubts that a burial depth of 1.5m would be enough to prevent post movement above the ground when there is a huge cantilever weight hung on the post. This is the reason that some of the following ideas are applied to gate posts:

Overhead post tie:



Substantial support structure:



Swinging gate support:



Back bracing into adjacent fence: