Nevermind.
A DIY site was selling the stuff you need for $10 a litre and recommending 7-10 litres per metre of wall plus equipment hire + time + repatching the walls afterwards, which for an entire house is prohibitively expensive just to get a HIA order lifted and a miniscule rise in rent. Add labour and its probably a $20,000 job for the whole house (around 30% of purchase price) since I can't DIY. SO not worth it. Is this why noone here has ever had a house treated?
Having now researched salt damp there's <2m of wall affected, the rest is just runoff damp without salt. I think I can live with a HIA order, and repainting that little bit of wall every few years. Surprising how little damage there was after ~15 years without any treatment, I've seen far worse on other houses.
This little tidbit is now in the column FOR keeping that house. HIA order will reduce the selling price but with only salt damp and boundary fences on it I can't see it reducing the rent too much.