Cut slope batter instability of overburden in Weavers Opencast Mine at Huntly involves 0-65m succession of Waikato Coal Measures and Glen Afton Claystone of the Te Kuiti Group (Eocene to Oligocene), and a 15-65m sequence of overlying gravels, pumiceous sands, silts, clays and peats of the Tauranga Group (Pliocene to Holocene). Field and laboratory investigations were conducted to determine the causes and mechanisms of cut batter instability. Field investigations included: detailed engineering geological mapping of the entire highwall at a scale of 1:1584; detailed batter logging and sampling of lower Tauranga Group materials and defect orientation surveys of Te Kuiti Group and Tauranga Group for stereographic analysis of failure modes. Laboratory investigations included: shear strength testing of the Te Kuiti-Tauranga Group unconformity contact and representative joint and bedding planes in the lower Tauranga Group; X.R.D. and chemical tests for clay mineralogy, grainsize distribution, atterberg limits, field moisture content, density and void ratio determination for Tauranga Group Materials.