Almost every question about odometer readings is really a question about which of three requirements is being asked about. They get merged into one, and then the answer is wrong in whichever direction the merge went.
Here they are, separated.
1. Per journey
Every journey recorded in the logbook needs an odometer reading at the start and an odometer reading at the end, along with the total kilometres, the destination, and the reason for the journey. Source: ATO, logbook method.
These apply during the twelve week logbook period only, not for the whole year and not for the four later years.
The readings are what tie the logbook to the physical car. A record of distances alone is a list of assertions; a record with readings is a chain, and the gaps in the chain are meaningful. A jump between one journey’s end reading and the next journey’s start reading is private travel, which is correct and expected. What should not happen is the readings summing perfectly to the period total with no private travel at all.
2. Per logbook period
The twelve week period needs an odometer reading on the day it starts and another on the day it ends, along with the start and end dates and the total kilometres travelled during the period.
These two readings do the arithmetic. End reading minus start reading is total kilometres for the period, which is the denominator in your business-use percentage. The work-related kilometres from your journey entries are the numerator.
Take these two readings on the actual days. They are the only two numbers in the whole exercise that cannot be reconstructed from anything else.
3. Per income year
This is the one people miss.
In each of the four years following the logbook year, you need odometer readings for the start and end of the full period you owned the car during that income year, plus your work-related kilometres and use percentage based on the logbook.
Two details worth being precise about:
It is the full period you owned the car, not automatically 1 July to 30 June. If you bought the car in November, that year’s period starts in November and your first reading is the one from the day you got it. If you sell in March, the period ends in March.
If you held the car for the whole year, the dates are 1 July and 30 June. That is the common case, and it is why the reading is usually described as a 30 June job. Practically, one reading serves double duty: your 30 June reading is also your 1 July opening reading for the following year, since the odometer does not reset overnight.
The 30 June and 1 July problem
The readings that matter most are the ones due on the two days of the year when nobody is thinking about their car.
A photograph of the dashboard with the date visible costs nothing and settles the question years later. Take it on 30 June, before you drive anywhere on 1 July. If you miss the day, take it as soon as you remember and note that it was taken late rather than adjusting the number to fit.
If you missed a reading
Do not invent one. A reconstructed reading is the sort of thing that looks exactly like what it is, and it undermines the readings around it that were genuine.
What you may have instead: a service invoice, a registration inspection, a roadside assistance callout, or a photo taken for some other reason. Any of these carries a dated odometer reading, and a documented reading from a nearby date with an explanation of why it is not the exact date is a materially better record than a plausible number with nothing behind it.
If a missing reading means you cannot substantiate a year under the logbook method, the cents per kilometre method may still be available for that year. It requires no logbook, though it is capped and you still need to be able to show how you worked out the kilometres.
What this looks like across five years
Year one, the logbook year: readings on every journey during the twelve weeks, readings at the start and end of the period, and readings for the start and end of the period you held the car.
Years two to five: readings for the start and end of the full period you owned the car, plus your work kilometres and percentage for that year. No journey readings, because the logbook period is over.
Whether the logbook survives all five of those years is a separate question, covered in how long a logbook stays valid.
Two things this page deliberately does not answer
Whether the 5,000 kilometre cap under the cents per kilometre method is apportioned when you only held the car for part of the year, and how balancing adjustments work when you dispose of a car or switch methods, are both outside what is set out here. They are specific enough that a wrong answer would cost someone money. Ask a registered tax agent.