25-year-old Yang finished at 15-under and two shots ahead of compatriot Mirim Lee, Lewis and Taiwan's rejuvenated Yani Tseng.
"I don't know what just happened," Yang said after sinking the final putt.
"I still can't believe I did it today."
After placing second behind Lydia Ko in last week's Australian Women's Open, Yang claimed the far richer Honda LPGA while Ko won the weekend's New Zealand Women's Open.
Karrie Webb was Australia's best performer finishing 9-under and tied for 11th. The only other Australian in the field, Katherine Kirk, struggled to reach par in each round and could only shoot 14-over and placed for 67th.
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