Nel was fielding near the boundary when the incident occurred

Pace bowler Nel complained to skipper Graeme Smith during the third day's play, and he later informed match referee Chris Broad.

There were similar problems during the first two Tests and Australia captain Ricky Ponting has urged fans to show South Africa "a bit more respect".

Ponting, who is playing his 100th Test, described racism as "unacceptable".

Following the incident, extra security staff were sent to patrol the section of crowd closest to where Nel had been fielding, but they were unable to identify the culprit.

Cricket Australia also ordered announcements to be made deploring racist abuse and similar messages were displayed on the scoreboard.

"None of the team minds abuse because it happens in any country.

"But we feel once it becomes a racist taunt then the boundaries are being overstepped and the South African team categorically deplores that type of behaviour," said South Africa coach Mickey Arthur.

"We made our point again and it has been reported and I'm sure it'll be dealt with through the right channels.

"I don't think we can do too much more other than just make our point that we categorically deplore it."

Arthur said Nel had been a target for lots of barracking during the tour, but most of it had been good-natured.

"Everyone boos when he comes on but I think he's going to leave Australia as one of the more popular guys. It's a bit of a love-hate relationship," he added.