The United Cricket Board of South Africa said the 25-year-old was caught early on Friday by Hobart police.
"The Hobart police released Nel into the custody of South Africa A team management as an alternative to proceeding with prosecution," the UCB said in a statement.
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It is not the first time the Easterns bowler has been in trouble. During the 2000-01 tour of West Indies, Nel was one of a group of players fined for smoking marijuana in their hotel.
Nel has played three tests, the last of them more than a year ago, and seven one-day internationals. He is due to take up a contract with Northamptonshire for the coming English county season.