A half-century from Sarah Taylor, who had struggled for form in the series, proved the bedrock of the England reply as they chased down a victory target of 123 with 15 balls to spare.
Taylor shared a 77-run stand with skipper Charlotte Edwards for the second wicket, while all-rounder Natalie Sciver and Katherine Brunt guided England home in comfortable fashion.
Australia had earlier been restricted to 122-8 from their 20-over allocation, with right-arm medium-pacer Sciver the pick of the England bowlers, returning 2-18 from her three overs.
Despite drawing first blood in the T20 programme, England still trai 8-4 in the points-based series and need to win the remaining two clashes to claim overall victory. Australia need just one win to regain the urn.