How many cigarette butts become litter in Australia each year?

This is a tricky one to answer because we need to consider a number of factors - including how many filtered cigarettes are sold each year, how many are smoked outdoors or in cars where they could potentially be littered, and then how often a person smoking outdoors will typically litter a butt.  Last time we estimated this the number, it was estimated around 3.5 billion butts are littered per annum.