I think they are making a new site for co-op, that's why the updating of the current site is a bit slow this year.
If you want dates send a quick email to
b.okane@unsw.edu.au , i an pretty sure she'll help you out there.
However suppose the interview dates are after your planned leave for holidays, you will need to seriously reconsider whether to postpone your plans or not. The reason they don't hold alternate interview days is because interviews are usually consisted of an uni academic and a person from a sponsor company, it would too much of a hassle to ask sponsor companies to come multiple days.
Co-op is one of the bigger scholarships out there, 15k a year is quite a lot and the experience you gain on industrial training would probably be more valuable for your future career than the money. It might be bothersome now to reorganise your trips but if you do get in, you don't have to worry about organising work experience or internships while you are at uni.
In the end, it's up to you.
EDIT: don't believe 2 people posted while I typed my post up...>_<....this is BoS...that's not meant to happen