I’m not overly familiar with the Victorian weapons legislation. But, my understanding is that something like this would be considered a ‘dangerous article’, on the basis it’s not within the list of ‘controlled weapons’. the basic rule with a dangerous article is that you must not have a dangerous article in a public place without a lawful excuse - such as it being a necessary part of your job, and you’re performing functions which require you to have it with you.
Anything you propel vertically, that is in the nature of a toy, or an experimental, rocket or aircraft - > less likely to be a problem.
Anything that you propel other than vertically, if it's not an obviously low powered toy, even if it doesn't look like a gun,
will almost certainly be a problem of some kind.
If we are talking rocketry, then you may care to do a keyword search of the CASA website for "rocket"