The code is no big deal - no permit or consent required, 150mm H5 rounds at least as deep as high at 1.2 centres and backed with 200x50 H4, with drain coil, sleeve and scoria back fill should do it.
I may be overly cautious but if your 30m was in a straight line I'd be tempted to put more posts in. Mitre10 quote 150 @ 1200 centres but most other examples are 900-1000mm.
When my neighbour had his walls done on a hillside site they saved on labour with the drainage by using recycled porous blocks of polystyrene behind the planking rather than 100% metal.
Yeah I think you are right, @Bung ... would hate the walls to start shifting - I think better to estimate on 900 centres for 150mm poles... or use 200mm poles at 1.2 centres and commensurately larger holes and more concrete. Will have to do the sums on that trade off...