The transition metal tellurium oxychloride, Ni-5(TeO3)(4)Cl-2, has been investigated by high-field electron-spin resonance for frequencies up to 3 THz, at temperatures well below the magnetic ordering at 23 K. At zero external field several resonance modes have been identified. When the applied magnetic field is perpendicular to both the a and b crystallographic directions, one of the magnetic-resonance modes softens, and a spin-flop transition occurs around 10 T. The results are discussed in terms of the crystal structure, and compared to other magnetically ordered materials with multiple magnetic sublattices, including orthoferrites and triangular antiferromagnets.