Generally speaking, if a party / function is held off the employer's premises and the cost per head is less than $300 the cost will be exempt from Fringe Benefits Tax (FBT). 

Section 58P Fringe Benefits Tax Act 1986 (FBTAA 1986). However if the exemption is claimed then a tax deduction for the cost is not permitted, nor can the GST component be claimed.

If the party / function does give rise to a FBT liability then a tax deduction will be available and the input tax credits may be claimed (if registered for GST).