I don't understand how you didn't foresee this happening... surely you realised how much it would impact your results?
You would have been much better off at least *trying*; even if you still failed, you probably would've gotten more than 18.
The honest answer is: yes, it's probably going to impact your average more than you'd like.
It will impact your GPA more so than your WAM, because WAM discounts the effect of units done in 1st year to half.
You're going to be pushing shit uphill for the rest of your degree just to get a decent average.
Even if you do get HDs for the rest of your degree, the highest your GPA can be is 3.7625.
That's a very solid and very good GPA, but it's going to be difficult to achieve.
Of course, you may have a saving grace... if you transfer to the single degree, it's *possible* that the IT unit gets taken out of your average. I really have no idea how it works, but I do know that I myself have two averages - one for my diploma and one for my degree. I guess it's plausible that you may end up with two averages as well... though exactly how they'd divide up your units I'm not sure... maybe your IT unit will end up with an average of its own under "BComm/BBIS", while all your commerce units end up as "BComm" since you'd be getting the credit for them transferred across. Honestly though, I don't know... maybe ask
Mao?? He's transferred from a double to a single... ask him what his transcript looks like.
As for transferring, you should be alright:
http://www.buseco.monash.edu.au/esg/agu/policies/selection-incomplete-qualifications.html"Double degree students wishing to transfer from the double degree into the Faculty-managed single degree component of the double degree will normally be permitted to do so provided their progress toward the single degree has been satisfactory."
"[T]he minimum interpretation of "satisfactory progress" would normally be no failures but a higher level of performance may be required depending upon the particular current degree program."
It sounds as though they're willing to ignore your IT unit when they consider you, though I would go to BusEco Student Services and ask them specifically about this policy if I were you. It also seems that so long as you don't fail any commerce units, they'll deem your performance as satisfactory and transfer you. Obviously though, I recommend you perform as strongly as you can so you don't give them a reason to hesitate when they make their decision.
As for the whole 'future employer' thing, I wouldn't be worried about it. People aren't robots... you'll obviously have to explain the fail when the time comes (probably in a job interview), but if you manage HDs for the rest of your degree, then obviously an employer who picks up your transcript will notice the stark contrast between the fail and the rest of your results and assume there's a reasonable explanation.