The IRD rule is:

a) No GST invoice = you cannot claim GST except for special circumstances:

See: IRD: GST on Special supplies

In this case there are two issues:

a) The contractor does not supply an invoice

b) The contractor is probably not GST registered

c) The situation is not a 'special supply'

As a consequence this entry may not actually be claimable.

If it was claimable, you would allocate the payment to a non-gst purchase. If you do not have an account set up for Non GST Purchases, here is how to do it:

  1. Cashbook > COA + Accrual > Accounts
  2. Add a New Account
    1. Account Name: Purchases - Non GST
    2. Abbreviation: A-PURNG
    3. Type: 'Expense'
    4. GST = 'No'
    5. Classification: 'PURCHASES'