Unbolt existing fence panels and remove. Use crowbar to break concrete around posts and remove.
Use post hole digger to dig 400mm deep holes for posts at each end of fence line and where gate will be. Evenly space other posts between these so they do not exceed 1.8m apart.
Place post in 1 end hole. Check top is 1 metre above ground and use spirit level to make plumb. If your fence borders a footpath or boundary, allow for 60mm between fence post and boundary line. Mix quick set concrete following pack instructions and pour into hole around post. Repeat for other end post.
Run stringline between end posts at bottom and along top corner of posts. Set other posts in position (see Step 3) so they meet these stringlines and are plumb.
Mark a point on each post 150mm and 760mm from top. On posts adjacent to where gate will be, mark a line 18mm from side of post. Cut rails to length. Predrill and screw top and bottom rails into posts using 100mm batten screws so the top of rails meets marked lines and end meets 18mm line at gate posts.
To support letterbox, cut length to fit vertically between top and bottom rails to suit width of letterbox. Cut a piece to fit horizontally between these to create an opening to match dimensions of your letterbox.
Cut pickets to length to fit around letterbox opening and to fit rest of fence run (see Notes).
At a gate post, place a picket against rail end-grain. Make flush with top of post and use nailgun to nail through picket to top and bottom rails.
Nail next picket on face of rail so its side is flush with the picket fixed in Step 8. Nail another picket to face of rails at other end of this run of fence. Stretch stringline taut between tops of these pickets as a guide for those in between.
Using a picket on edge as a spacer, place next picket so its top meets stringline. Nail into top rail. Use spirit level to make plumb, then nail into bottom rail. Repeat for remaining pickets, including pickets around letterbox. Occasionally measure fence where you have yet to attach pickets and calculate how many will fit. You may have to adjust space between pickets so you don’t end up with an uneven gap at end of run.
Construct a gate frame 10mm narrower than gate opening. Make frame square, then cut and fit a diagonal brace to sit inside frame. Attach 200mm T hinges to gate frame so diagonal brace runs from bottom hinge up to latch side of gate. Screw gate frame to post, then nail pickets to gate frame to match rest of fence. Attach gate latch to gate frame and adjacent post.
Slide in and secure letterbox. Fill all nail holes and sand smooth. Apply 2 coats of semi gloss acrylic, allowing to dry after each coat. Spray painting is the easiest way to do this but you can use a small roller and brush.