England held South Africa to 111-9 from their 20 overs after winning the toss and choosing to bowl at Chelmsford, with Brunt claiming a career-high 4-15.
With those dismissals, the 37-year-old passed 100 T20 international releases and tied Anya Shrubsole for the most by an English women’s player with 102.
The visitors, who were missing their star all-rounder Marizanne Kapp because she had to return home for personal reasons, once again put up a below-par batting performance despite Laura Wolvaardt’s best score of 55. This has been the case throughout the white-ball phase of the trip.
Although they lost Danni Wyatt and Bryony Smith due to being more aggressive on the powerplay, England’s 50-2 victory was a step in the right direction, according to captain Heather Knight.
Dunkley, who was opening for the first time in international cricket, hammered a 39-ball 59 to break the back of the chase after Nat Sciver was out for only 13.
Knight led England to victory with 30 balls remaining with an uninterrupted 29-run partnership with Amy Jones.
The best bowler for South Africa was Ayabonga Khaka, who finished with 3-13.
The victory gives England an unbeatable 10-2 lead in the multiple-format series, with T20s remaining on Saturday at Worcester and Monday at Derby.