Pebbles from beach...illegal?

How many are we talking here?

I guess any amount in excess of a bucket-full; yes. Note I'm guessing here, but I'd imagine there would have been concerns sometime somewhere re coastal erosion if people were using diggers/carrying away trailers of sand.
 
Is it illegal to take pebbles/stones from a public beach?

As far as I can rember, you need a foreshore licence to remove any sustantial amounts of sand etc from the beach from below the high water mark.