As for developments today for Australia: Those in Metropolitan Melbourne and Mitchell Shire will be required to wear a face-covering when leaving home from Thursday. There will be fines of $200 for those who do not follow this rule.
I've been interested in the discussion around masks since the US CDC (who had had similar guidance to Australia) flipped to recommending them (back in June?). I suspect it's a good idea in indoor spaces where consistent distancing is difficult, like the recommendation here in Victoria last week (?). However, my impression has always been that outdoors is considered much lower risk of transmission than indoors, plus in my experience it is usually much easier to keep appropriate distancing outdoors, and so I feel a blanket "outside the home" rule is probably going too far.
Particularly for exercise, while no activity is completely risk free, I have throughout assumed that it is very low risk when done solo and trying to keep distance from those encountered briefly on the way. It is also much more likely to be an activity where wearing a mask is (in their words) "impractical" or, in my opinion, at least a more significant restriction than wearing it in a supermarket, and with much less benefit to the community.
One practical implication for me is this: I walk (or sometimes run) to the grocery store. If I understand correctly, masks worn for a period of time get a bit wet and become less effective, and that probably happens faster with exercise. If so, wearing a mask to/from the grocery store would surely make that mask less effective
in the grocery store - the place where I actually want it to be effective.
Oh, and one final concern is the possibility of masks making people feel safer than they actually are, and thus distancing less. If you are wearing a mask and can keep further away, do (even the magic 1.5m rule isn't about guaranteeing you are safe, just about reducing the risk that you'll catch anything).