Queency - In the email invitation I got, they specify the dress code, and specifically say for women to wear a "knee lenght skirt/dress with skin coloured stockings".
That is for Melbourne, and it never gets too cold here, so it may differ there, but I'd be guessing that they'd still ask you to wear the same thing so that they can get an idea of what you'd look like in the uniform.
Vix - I got an invite to the assessment day a month before it was on, but from what I've read, many people just turn up & that isn't a problem (just make sure you take a CV and photo).
Where I live, the assessment day is in the CBD, but I think it differs a little bit city to city