is throwing ash considered littering?
Technically, yes - but in reality, it's mostly ignored.
Section 144A of the NSW Protection of the Environment Operations Act 1997 defines
litter as:
(a) any solid or liquid domestic or commercial refuse, debris or rubbish and, without limiting the generality of the above, includes any glass, metal, cigarette butts, paper, fabric, wood, food, abandoned vehicles, abandoned vehicle parts, construction or demolition material, garden remnants and clippings, soil, sand or rocks, and
(b) any other material, substance or thing deposited in or on a place if its size, shape, nature or volume makes the place where it is deposited disorderly or detrimentally affects the proper use of that place,
deposited in or on a place, whether or not it has any value when or after being deposited in or on the place.
Since ash is a solid material, it is covered by the above definition. The offence of littering then follows at section 145(1):
145 Littering generally
(1) Offence of littering A person who deposits litter in or on a public place or an open private place is guilty of an offence.
Maximum penalty: 20 penalty units.
One penalty unit equals $110, so that's a max penalty of $2,200.
I wouldn't be concerned about it though, because there's no evidence - the ash is blown away almost immediately. You'll only get done for something like this if a bored cop actually sees you do it, or if there's photographic or video evidence. Someone whinging about it won't be enough on it's own.