I'm Australia like you, so Don't know if this helps. Our horses are out on pasture 24/7 also. They get fed one small scoop of a mixed feed daily, and this is only so they can get Apple Cider Vinegar which my pony has to be on at the moment. This is all they get, and even when in heavy work we only increase the size of the scoop. They are all fat, except for whichever one happens to eb in full work at the time, and he will be fit looking, buu nowhere near skinny.
In regards to hay. I feed Lucerne, simply because my horse is on pasture, why would I give him pasture hay when it's already out there? BUT, we make sure to buy lucerne that is much more stalk than leaf. This ensures that the hay is more fibre/roughage than the rich and sweet leaf of the lucerne. It takes them longer to eat, they need to chew it more, and all round we find it better for our horses. The only time we feed ours pasture/meadow hay is when they are OFF pasture.
If you horse is on decent pasture 24/7, I would suggest that you could cut the chaff. She is getting some hardfeed as well correct? I would think that the hardfeed would be enough to replace anything she may have lost throughout the workout.