hi there.
i know everyone sings the praises of the co-op program, but i don't think it's that great. the best thing is that it offers varied experience, and you get paid while at uni, but i personally think that students get a bit of a raw deal.
$15k per year (or whatever it is) works out to about $250 per wk after tax. this is pretty sweet while you're at uni doing nothing, but it feels pretty rough working 35-40 hours a week during the holidays for only $250.
The whole experience is also a little bit of a jackpot depending on your employer - some are fantastic, offering you great working conditions and interesting work - while others will basically treat you as a slightly glorified office assistant.
the civil engineering co-op isn't as well established as some of the other co-op streams, so the range of employers won't be the best either.
there are heaps of better scholarship/cadetship programs out there with much better conditions and less responsibilities. (sorry, i'm not up to speed on civil engineering opportunities, but have a look around).
good luck with the HSC and your application for uni!