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For VFR flights, your SARWATCH is based on a SARTIME nominated by you when you submit your flight plan. There would be no point in cancelling SARTIME on a CTAF frequency because no air traffic service is listening on that frequency. You would simply be talking to other pilots.
You are asked to use a telephone to cancel your SARTIME when ever possible on 1800 814 931. Otherwise, you may cancel SARTIME by radio with the ATS unit responsible for your area - but certainly not the CTAF.
Read up on ENR 1.1 para 8.2.2 for details on cancelling SARTIME. Also AIP ENR 1.10 para 2.3.