I don't think it does, but then it depends which way you look at the figures. Firearm deaths had been trending down before Port Arthur, however since then there have been no massacres and the rate per hundred thousand has reduced. Raw numbers should be considered against population growth.
How much signifigance do you place on a downward trend as you would have to think the reduction would have been likely to stabilise at some point, also spouse deaths showed a reduction and the suicide rate from firearms does appear to have reduced. There are a number of conflicting statistics you can find proponents that will say there has been a 50% reduction in deaths on one hand and no change at the other extreme. I am firmly of the view that the Howard changes resulted in less deaths I am also aware that if you go looking you can find internet references that in some cases don't agree and others that do. It is difficult to filter out the distorters of the truth and the mistaken it would be nice if someone with full access to reliable statistics could provide something more acceptable to all.