Abstract: Wanganui Basin,located between North and South Island of New Zealand,is characterized by thick crust and low heat flow. Previous study attributed its formation to bending of thin plate under vertical component of drag force acted by the subducting slab on the overriding crust. The present study shows that the flexure hypothesis of thin plate is not self consistent. We propose that the process of underplating is responsible for both the sinking of Wanganui Basin and the rising of Axial Range east of the basin. This model can explain the synchronism of sinking of basin and rising of mountain,the southwest migration of sedimentation center of the basin and uplift of the once subsided northeastern basin,and the low heat flow observed in the basin.