Does an employer have to pay holiday pay to its casual workers?

Yes. Every worker (whether an employee or otherwise) is entitled to paid annual holiday, and (as is common, given the short-term nature of casual workers' engagements) accrued holiday pay on termination, calculated from day one of their contract.

For holiday years beginning on or after 1 April 2024, there is a new method for calculating holiday entitlement for casual workers who meet the definition of "irregular hours or part-year workers". Their entitlement will accrue throughout the year, at the rate of 12.07% of the actual hours that they have worked in each pay period. A pay period depends on how frequently the worker is paid.

Employers can choose to implement rolled-up holiday pay for these workers. 

Not all casual workers will fall within the definition of irregular hours or part-year workers. Holiday entitlement for those workers that do not meet the definitions (and for all workers where the holiday year began before 1 April 2024) remains 5.6 weeks per year. 

Holiday pay should be based on the worker's average weekly hours over the previous 52 weeks (or, as is likely for a short-term engagement, since the beginning of the engagement), discounting weeks in which they received no pay or were absent due to sickness. Where possible for longer engagements, the calculation should take earlier paid weeks into account to bring the aggregate up to 52, but should not go further back than 104 weeks.