This one from here was one of those.
The cannon in question is actually at Williamstown, across the Yarra mouth from Melbourne and somewhere we've been for the odd day out. I'm glad to know the cannon is rendered safe.
It's a good example of how a great little story can be told on film with minimal props. (Obviously, in that case you don't use your own cannon, but 'borrow' one.)
There's actually an irony over the apparent humour of this idea, as there are still thousands of unexploded bombs across Europe and the UK, many buried under the cities. Likewise some display bombs, cannon and missiles have been found to be not quite as safe as was thought. However most of the taller stories are urban myths, of course.