There's no way of specifying only half a day in the time sheet when entering the sick leave, but after creating the pay if you edit the sick leave pay line you can change the number of days to 0.5.
The Holidays Act 2003 describes the sick leave entitlement in terms of days, and does not make provision for dividing it into smaller units, such as part days or hours. This means that if an employee works for part of the day and then goes home sick, this will be counted as using a whole day of sick leave, no matter how much of the day the employee has worked prior to going home.
However, it may be open to an employee and employer to agree to express the entitlement in terms such as hours or part days if this results in providing the employee with a better entitlement than the minimum set out in the Act. Payment for this sick leave day would be based on their relevant daily pay.
See the Dept. of Labour website for more information http://dol.govt.nz/workplace/knowledgebase/item/1264