Involved in the biosynthesis of kunzeaol. Produces mainly (-)-germacrene D along with gamma-cadinene. (2E,6E)-farnesyl diphosphate + H2O = diphosphate + kunzeaol Binds 3 manganese or magnesium ions per subunit. kcat is 0.03 sec(-1) with farnesyl diphosphate as substrate (at pH 7.5 and 28 degrees Celsius). Secondary metabolite biosynthesis; terpenoid biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsa subfamily.