Southern Cross says it's basically a cost thing. To avoid large-scale premium increases, only drugs listed on the Pharmac schedule qualify for cover on most plans and benefits.
"Pharmac has to achieve a balance between getting good health outcomes for New Zealanders and ensure pharmaceutical costs do not spiral out of control. Southern Cross has a similar objective. Adding expensive drug treatments to our benefits would increase premiums to the extent they would become unaffordable for many members."
You can find policies that pay for non-Pharmac medications, such as from Accuro or nib, but most require that the medication be prescribed by a specialist.