When a financial arrangement ends, for example you close your foreign bank account, you must do a base price adjustment (BPA) whether or not you're a cash basis person.
How the BPA works
The BPA is a final calculation to ensure no income or expenses are missed or counted twice.
If the calculation is positive, this is income. If the result is negative, the negative amount is treated as interest expenditure. Whether it is deductible depends on the normal rules.
Large currency changes or smaller changes on large amounts may result in income where a financial arrangement is not in New Zealand dollars.
Calculate your BPA
To complete a base price adjustment, individuals who are a cash basis persons can use the Sale or disposal of financial arrangements - IR3K form as part of filing their tax return.