We have used high-resolution neutron powder diffraction over the temperature range 9-65 K to determine the magnetic structure of ErFe_11.5Ta_0.5. Above 49 K the magnetic structure is a collinear ferrimagnet with an easy c-axis. Below 49 K, the magnetization cants away from the c-axis and eventually reaches a canting angle of 9.0(1.3)° from the c-axis, as T0 K. This behaviour may be understood in terms of the competition between different order terms in the Er³⁺ crystal-field Hamiltonian: the spin-reorientation away from the c-axis is driven by the 6th-order crystal-field term.