use a common holiday anniversary

Good morning 

We have run our MYOB payroll for years with a common anniversary date of 23 Dec as we have a three week annual shut down at Christmas. 

My accountant has said a common anniversary date is not allowed as it does not compliy with the NZ Holidays Act 2003 and cited below.. 

All employees, regardless of their classification (ie including part time, full time, fixed term and 'casual') but not including the Armed Forces, are entitled to at least:

four weeks of paid annual holidays (sometimes referred to as annual leave) after each 12 months of continuous employment for their employer, for rest and recreation.

To change employees over to the annivesary of their start date and work out their leave balances / Holiday pay will require some work. 

Could some  one please confirm with supporting evidence that a common anniversary date is actually permissible?? 

Many thanks.