I am curious--do kids actually start mid year ON their birthday? or did you mean they would start in January when aged 5? (hoenst question--I don't know how thigns work in NZ)[/QUOTE
They start on their birthday (or the next school day) all through the year.
At my daughters school they do a 10 week school visit program so my DD who will be turning 5 in September goes every Friday until then, when she turns 5 she will start school in year 0.
Then next year she will be in Year 1. For those whose birthdays are before the end of May then they start in Year 1.