In brief
The aim is not the biggest possible deduction. It is to claim what you are entitled to and be able to explain the result.
Apply the business-purpose test
Software used to invoice clients has an obvious connection. A family phone plan, weekend vehicle use or a trip combining work and a holiday needs more care. The answer may be a reasonable business percentage supported by evidence, not simply all or nothing.
Record how mixed use was apportioned
Keep a diary, usage record, logbook or calculation showing how business use was separated from private use. Repeating last year’s percentage without checking whether circumstances changed is not a strong method.
Avoid double counting vehicle costs
Eligible sole traders and some partnerships may use the cents-per-kilometre or logbook method for a car. The 2025–26 cents-per-kilometre rate is 88 cents and already covers running costs, including decline in value. Fuel, registration and depreciation cannot then be added again under that method.
Choose a working-from-home method that fits the records
The 2025–26 fixed rate is 70 cents per hour and covers specified running expenses. Keep a record of actual hours worked from home for the full year and evidence that the relevant costs were incurred. The actual-cost method may suit detailed records, but the method must fit the facts.
Separate an asset, its finance and its running costs
Computers, machinery, vehicles and other assets may be subject to depreciation or specific small-business rules. Record the purchase date, first-use date, total cost, trade-ins and finance. Asset cost, loan principal, interest and running costs are different accounting and tax questions.