The genus Ambolus was proposed by Ikeya et al. in 1998. Two Ambolus species, A. pumilus and A. coniunctus, from Australia have been described thus far. Here we describe four new Ambolus species, A. swansoni sp. nov., A. stewartus sp. nov., A. delicatulus sp. nov. and A. cavallius sp. nov. from New Zealand. To clarify the stability of their carapace characters, we focused our attention on the ontogenetic differentiation of the pore pattern and the shell ornamentation of the above six species. Based on these findings we demonstrated that A. delicatulus sp. nov. differed from the other five species at the A-2 moult stage. Ontogenetic differentiation of the shell ornamentation suggests that although the two Australian species are closely related and the four New Zealand species also resemble each other, these species differ in certain aspects.