no. a bachelor in eng and a master in eng are exactly the same thing in the case of the melbourne model. both are recognised by engineers australia (the governing body for engineers) and both will lead to a career in your chosen field.
More importantly, the 'double degree' of the melbourne model isnt as flexible as doing a more conventional double degree at another uni. This is because you are restricted by what you can major in at melbourne in science ie to be able to do master of engineering in civil engineering you MUST major in civil systems in the bachelor of science at undergrad level, then do your masters. However, doing a BSci/BEng at, say, Monash, in science you can major in any subject field, as well as majoring in your chosen engineering stream in the engineering degree.
Btw, the 'prestige' of melbourne uni is perceived by the public, but does not extend totally to engineering by employers. RMIT and Monash both are regarded as slightly higher by graduate employers than Melbourne (thats not saying its a crap uni!) Its just that the aforementioned uni's teach engineering in a way that employers like. That is, using a common example, Melbourne will drill into you how to build something with theory, tests, exams, examples etc. whereas RMIT and Monash will actually get you to build it. Employers seem to like this way of teaching.