You are likely not getting it hot enough to burn off the carbon (coke) deposits that have built up over the past 20,000 km. They ignite the residual fuel, causing run-on. If it is just a second, there isn't much to worry about, it is just an annoyance.

You could try using a one-time combustion chamber cleaner, but it is a toss-up as to whether the cure is worse than the disease.

We would call it run-on in North America, not sure about Oz. Overrun is defined as the engine speed being higher than the throttle setting, ie when you are going down a long hill and using a lower gear to slow your speed.

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Retired:
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