A deck of that height will need bracing - and that means in each plane - triangles provide strength other shapes (squares & rectangles)are inherently unstable. Add some timber or steel strap bracing from top of one post to bottom of adjacent one and vice versa is best, but they can be braced with shorter pieces of timber too. 45 degrees is ideal, but that can be varied to suit the specific structure.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.