Her birthday was the next day - Sunday.
I also think you need to factor in people have different approaches to backpacking.
I travelled both alone and in a group. I would not head to the bar/common room and tell everyone it was my birthday whereas it sounds like you would.
Some of my friends preferred to avoid hanging out with backpackers at all costs! Some like to stick close to the travellers trails and hangouts. Some people like to party. Some people are more focussed on seeing the sites.
Generally speaking, if you can meet some locals, this can open a lot of doors to better experiences and especially save you a hell of a lot of money. Hell in my day (20 years ago) we never had the kind of money these kids have. So sure I could afford a beach hut in thailand at US$10 per night but a place like London you dossed via networks of mates until you could find a job and hooking up with a local was a big plus!
So I am not really surprised if UK backpackers would seek out locals early on
I guess we are going to learn a lot more about who she was connected with and what she had done so far
I'm not the sort of person to walk into a room of strangers and tell everyone it's about me. Plenty of backpackers are loud mouths who love attention but that isn't me. I don't especially try hard to get along with fellow people in the hostel either. If anything I'm a loner. I don't appreciate the implication of 'it sounds like you would.' Kinda harsh.
She'd been at the hostel for a few days, she'd spoken to the people in her dorm. She'd also travelled to the Bay of Islands a few days previous. Hostels are transient places and your allies can be afternoon-long. But I think it indicates she had some pals there.
So, at some point you'd probably mention it was your birthday soon and people would be like 'oh come on, we've gotta do something, it's your birthday! How many birthdays are you gonna have in Auckland?' Not to mention her adding people on facebook and the platform's function of alerts for your friends' upcoming birthdays.
As for 'it was her birthday on Sunday, not Saturday...' People are going to go out on a Saturday if their birthday's Sunday – clubs heave more on a Saturday night, there's more excitement on a Saturday night. Most young people consider that their birthday night, and it falling on a Sunday means you're chuffed as you can go out Saturday for it. And when you're backpacking, any excuse is a good one.
She also had a heap of money. She didn't need to worry about saving cash because she was at the start of a (year?) long trip. Her dad's also got money. She probably did the hostel thing as a way of meeting people and to live that lifestyle. The 45 bucks she could've said by staying at someone's house for a few days probably didn't enter the equation.
I just find it weird that someone considered popular, funny, and friendly (though these terms are quite often used in these cases) would go out with a random guy on a Saturday night when she's been at a hostel for a while.
Then again, so many times you're keen for a night out but no one else is. Sometimes it's just the combined vibe. When you're travelling, Saturday nights aren't more alluring than any other and they mean less. Everyone could have been partied out but she wasn't. I'm just raising questions.
Anyway, most of this conjecture is pointless, because so often these details completely derail threads.