Du Plessis, who has accepted the sanction, showed dissent by "occupying the crease for a period of time" after being given out lbw before leaving the field while looking at his bat and shaking his head, the ICC said in a statement on Tuesday.
It was Du Plessis' second sanction for dissent in 12 months, after being fined 15 per cent of his match fee against India in a one-day international in October.
Du Plessis wants under-performing bowlers to shape up
A third offence over the next seven months would result in a suspension for the 31-year-old.
South Africa failed to teach the World T20 semi-finals.
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