In coastal areas, it's a fact of life I'm afraid and good airflow around your plants is the best you can do for them. Would have thought averaging a 20% loss around Sydney, especially with some of the storms they can get there at times, is fairly normal.
You've got them in the open, so airflow's about as good as you can get. I'd be iffy about putting a roof over them without ventilation as it would just trap humidity where you don't want it. One thing I'd watch for while they're in flower is storms, as hail in particular, and even heavy rain, can damage the buds creating a good spot for mould to start, so if you can cover them or move them under cover at those times that might help. Also keep a close eye out for insects and caterpillars, as they can also cause damage creating a start point. Sounds like you've been getting some shitty weather down there this season, so that won't help.
I'm in a semi arid inland area, and I lose a few buds outdoors each season to mould, so it's just something we all have to deal with to some extent. I wouldn't be blaming coco though, unless you're using shit quality, or not giving them the right feeding regime which could leave them weakened.