However, the 2004 Athens gold medallist was only third quickest overall with his time of 1:50.20 seconds behind that of the impressive Chad le Clos (1:49.83) and Sebastien Rousseau (1:50.03).
Townsend is the only swimmer of the three that has already dipped under the A-standard qualifying time of 1:47.82 for London 2012, but needs to swim it for a second time in the final of the event on Wednesday.
The SA Sport Confederation and Olympic Committee (Sascoc) requires all its Olympic hopefuls to swim below the qualifying times twice during a 10-month period, including once this week.
The semi-final of the event takes place on Tuesday.
Elsewhere, Charl Crous and Darren Murray, both without A-standard times, looked good as they finished one-two in the 100m backstroke.
Crous was a little over a second slower than the benchmark time of 54.40 with a 55.84, while Murray swam 56.09 to book his place in the semis.
In the women’s events, Karin Prinsloo, who has one A-standard time in the 200m backstroke, impressed in the 100m of the same event when she was quickest through the heats.
The Northern Tigers swimmer won her heat in 1:01.53, ahead of Lehesta Kemp (1:03.99), while Jessica Ashley-Cooper (1:04.88) was third.
Another athlete with an A-standard time, but in a different event, Kathryn Meaklim, qualified for the 100m breaststroke semi-final in 11th place after a time of 1:14.45.
Meaklim, who booked herself a ticket to London by winning the 400m Individual Medley on Monday evening, was well behind top qualifier Suzaan van Biljon (1:09.83), with the latter likely to give it her all in what is her final chance to make the Games.
Finally, Wendy Trott, the swimmer with an A-standard time in the 800m freestyle, was quickest in the 400m free heats as she recorded a time of 4:13.49.
Jessica Pengelly was second fasted in qualifying for the final after a 4:19.30, while Roxanne Tammadge (4:19.71) was third.