The win makes it her third overall singles title and second for 2018 following her outstanding run at Nottingham in June.
Wang served seven aces and converted five of seven break points against the overmatched Muguruza to become the first Chinese player to reach the final of the tournament.
Asked in her post-match press conference about her preparation for the two unexpected matches, Wang said: "Well, I'm not prepared".
Barty took a 4-0 lead after winning 16 of the opening 22 points in the match but Wang, who was playing her fourth WTA final in as many months didn't back down, winning three consecutive games.
Madison Keys has withdrawn from the WTA Elite Trophy because of a knee injury.
Barty's week had begun with defeat to Aryna Sabalenka in the Orchid Group, but she battled back to beat Caroline Garcia and qualify for the semi-finals.
Facebook blocks 115 accounts for alleged 'inauthentic behavior' ahead of midterms
Employees work in Facebook's " War Room ", during a media demonstration, October 17, 2018, in Menlo Park, Calif. It added all the Facebook pages associated with the accounts appeared to be in French or Russian languages.
The two-time grand slam victor from Spain won her final round-robin match - effectively a playoff for a semifinal spot - 6-7, 6-2, 7-6.
Barty took full advantage, taking down world No.14 and defending champion Julia Goerges in the semifinal before beating Wang in the decider, collecting A$828,000 for her week's work. The Frenchwoman needed to win in straight sets losing no more than seven games.
"I was just called upon yesterday and I won yesterday".
"My goal is to stay top 20 and try and consolidate as best I can".
However, it was the Australian who came out on top to take the only break of the second set in the seventh game, before serving out her second tournament win of the season after her victory at the Nature Valley Open in June.
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