I should really know the answer to this question, as I am in HR but the NES are vague regarding this issue.
Background: I have been employed by my workplace for 8 months on a part time basis. I have accrued annual leave and at the beginning of this year my boss stated that she wanted to control our annual leave a bit tighter to ensure that we weren't short staffed in the office. There are 3 part time employees (including me) and 1 full time employee. Operationally we are not hugely significant in the organisation (so if there was only one person in the office there would not be an issue). Thinking that I would do the right thing I applied for the annual leave that I knew I would have to take this year. 1 day in May for a family function, and 3 days in June for a wedding in Fiji that I am bridesmaid for.
My boss has come back and approved my one day in May, but has refused to approve my June leave until at least May at the earliest so that she is sure what is going to be happening around the office at that time!! Nobody else has booked leave for those 3 days and I need to start booking flights and accommodation so I can't wait until 2-3 weeks before to get it approved. At the end of the day I will go anyway, approved or not, as it is my best friends wedding and I agreed to be bridesmaid a year ago!
Anyway, the NES says that employers can't reasonably refuse annual leave requests, and seeing that I have only requested 3 days off - 6 months in advance - when nobody else is on leave, I feel that they are being unreasonable. But when I spoke to them they said they hadn't refused to approve my AL, they had just refused to approve it NOW, and I needed to wait.