Soft washing is chemistry rather than force. A biodegradable solution, usually sodium hypochlorite with a surfactant to help it cling, is applied at close to garden hose pressure, left to dwell, then rinsed. Nothing is blasted. The growth dies and weathers off over the following days.
Why it lasts longer
Mould, algae and lichen have root structures that sit below the visible surface. High pressure removes the top layer and leaves the roots, which is why a pressure washed wall often looks green again within three to six months. Killing the organism at cellular level typically buys twelve to twenty four months instead.
On the coast that window shortens. Salt and humidity around Merewether, Bar Beach and the lake suburbs give spores a head start, so expect the shorter end of that range close to the water and the longer end inland.
Where it is the only safe option
Painted render, weatherboard, fibre cement, Colorbond and roof tiles should all be soft washed rather than pressure washed. High pressure strips coatings, forces water under laps and into ceiling cavities, and on older tiles it removes the cement skin that was holding growth off in the first place.
One genuine risk worth naming: sodium hypochlorite can react with organic pigments in some paints, particularly reds, blues, greens and oranges, and staining from that cannot be washed out. We test an inconspicuous patch before committing on any painted surface.