You can claim any business expense which is linked to the production of your business income.
GST cannot be claimed on any part of your overseas trip.
Your accountant and/or Inland Revenue may ask for proof that your trip was indeed for business purposes, so retain copies of invoices or tickets to support claiming of the expense.
If you have a trip which has part business, part pleasure, then simply claim the relevant percentage or the direct costs of that part of the trip where you can identify the link to income production. For example, if you fly to Fiji and spend a week on the beach and a week seeing clients or suppliers in Suva, then claim all the Suva expenses and half of your travel costs to Fiji, such as the air fare.