Casual Employees are not specifically mentioned as a category on the WINZ subsidy form or in the Government's explanation.
In terms of the subsidy you can apply for your casual staff also. Average the hours they have done for you in the last year and apply on that basis. Like any staff who are calculated to work less than 20 hours per week (the $350 per week subsidy) they should not be benefitting from anything extra from the subsidy. If their average works out to be $200 per week pay them $200 from the subsidy and retain the balance.
We would expect to see an MSD audit once all this is over so it is important to remain accountable for the subsidy value in it's entirety. If you are not passing it on to staff for whatever reason you should certainly retain it as you will be expected to return it.