WAM=Weighted Average Mark.
First year units are weighted 0.5 and everything else is weighted 1. So that is first year units are worth half of other units. With the weightings taken into account it's basically a (weighted) average of the scores for all your units.
GPA=Grade Point Average, same weights as for WAM but instead goes on the grade level you get. So HD is 4.0, D is 3.0, C is 2.0 and so on. I guess the difference between this and WAM is that you might have a WAM greater than 80 but have a GPA that isn't 4.0 (that is not HD's in everything), as you could get 100 in one unit and 79 in another.
HWA=Honours Weighted Average.
Before this year, you would only graduate with honours for engineering if you averaged a certain score (used to be 75+ for first class, and something for the lower ranges). Now as they've changed the system and are giving everyone honours with their engineering degree, you'll still get honours, but have to average higher to get the same class of honours, so now it's 80+ for first class honours. This means that some people may graduate with honours in engineering as they are entitled to, but without any class of honours (if they get between 50 and the cut off for the lower grade honours). I should also note that the HWA only takes into account your engineering units. That is if you say are doing a double degree with science, if you average 82 overall but in your engineering units only average 75, you won't get first class honours. Also the years are weighted differently for HWA, basically first year is weighted 1, second year 2, third year untis 3 and fourth year units 4. For me I think it worked out to being something like 40% of the score goes on the final year units (because of the way I've got it structured with the double).
As for which one matters the most? For engineering, a high wam/gpa will get you past the first stage when applying for jobs, that is getting selected out of the piles of applications through automated processes. After that a lot more goes on what experience or vac work (which now isn't compulsory...) you've done, the Honours class might help a bit, but a lot of emphases goes on experience (for engineering anyways), from what we've heard so far.
So yeah, a high average will get you past the first application stage, but experience plays a large role after that.
Note: I haven't started applying for positions yet but this is what we've heard from older members and through various lecturers and such.
Here's something I made a while ago. It wasn't perfect at first and I can't remember if I updated it since, but it should give you an idea about it all (just ignore the unit handbook links for the wrong year).
WAM, GPA (4.0), HWA Calculator (MONASH)
EDIT: Since that's probably still confusing I'll just summarise it a bit.
WAM - Weighted Average Mark
First year unitsweighted: 0.5
Other years units weighted: 1
Takes into account unit marks, e.g. 75 82 e.t.c.
GPA - Grade Point Average
First year units weighted: 0.5
Other years units weighted: 1
Takes into account the grade bracket you fall into - HD (4.0), D (3.0) e.t.c.
HWAUnique system for engineering to calculate which honours grade you fall into
First year units weighted: 1
Second year units weighted: 2
Third year units weighted: 3
Fourth year units weighted: 4
Takes into account unit marks.
For a standard 4 year engineering degree with 8 units in each year level this gives for the HWA:
=80)
First year

Second year

Third year

Fourth year

Remembering that the HWA is more latter year heavy where as for WAM and GPA your first few years count more than the above.