Most of the restaurants we frequent in suburban Sydney (and there are an embarrassing number that consider us regulars) are happy to box up leftovers and will usually offer before we can ask. These include Italian, Indian and various Asian cuisines; places which are not really "fast food" but offer takeaway as well as full table service, so they have the containers available. At Asian and Indian restaurants, RadioBloke and I deliberate over sharing one main course (and finishing it) or two (and taking some home).

One (higher priced, unique dishes, owner as chef) Indian restaurant stopped allowing patrons to take leftovers home on the grounds that improperly stored/reheated food could cause food poisoning. I guess he'd had legal advice. Numerous other restaurants in the same street still allow it, so it certainly isn't a local regulation. One place charges 50 cents for the container.

We haven't asked at some (steak places, for example) where we usually finish our own meals. We don't often eat at the really top end 5 star places, but IME the portion sizes are more appropriate and so the issue hasn't come up.

When I'm not sure of the restaurant's policy and can't finish the meal, I'll say something to the waiter like "that was really delicious, but unfortunately I just can't finish it all" and wait for them to offer to let me take it home.