Studies in Mahurangi Forest, northern New Zealand, suggest that criteria for assigning soil profiles to the five soil drainage classes currently used in N.Z. need revising. Two hydromorphologically distinct profile types were identified among Ultisols currently classified as imperfectly drained. The wetter profile type is characterised by the presence of low-chroma mottles (≥2% but <50%) in the upper profile (0–30 cm) whereas the drier profile type contains only redox segregations (≥2%) in the upper profile. Most of these imperfectly drained soils were classified as Mottled Yellow Ultic Soils in N.Z. Soil Classification (NZSC), i.e., despite the hydromorphological difference, both profile types were considered to be the same at subgroup level of NZSC mainly because they were assigned to the same drainage class. However, we found that the two profile types were significantly different (P < 0.05) in terms of six plantation forest management-related physical and chemical topsoil properties (macroporosity, total C, Bray P, Bray K, Bray Mg, and pH). On the basis of these differences the two profile types could potentially be managed differently and hence we propose that the current imperfectly drained class be split into two new classes (giving six drainage classes in all): ‘imperfectly drained’ soils (drier profile type) and ‘somewhat poorly drained’ soils (wetter profile type). The imperfectly drained subgroups of the NZSC could be distinguished taxonomically from the somewhat poorly drained subgroups if the rather impractical permeability classes of the soilform (the fourth level of NZSC) were replaced by the modified soil drainage classes.