Unfortunately I was using my Auckland university email at the time and that account is no longer available to me since I don't work for them any more (funnily enough). This means I can't check the email time stamps to give you a good answer.
I think I had about one month or maybe more to book my flights but Martha did call me to ask if I could change my interview day to make room for someone coming from overseas about 4 days after I was originally emailed. Although my interviews were a little later than originally planned because of trouble getting all the panelists together.
I'd say you can either email Martha and ask her and she might be able to give you a rough idea or go the 'surprise' way and wait and see when you get an email
I heard recently over 200 people applied last year for the 20 spots available. I'm not sure if there are 20 other's in the class though. I can count about 12 by name :unsure: