Two tick species Aponomma hydrosauri and Amblyomma limbatum that infest large reptiles have an abrupt parapatric boundary near Mt Mary in South Australia. A previous model has suggested that the boundary is maintained at population density troughs resulting from habitat heterogeneity along a gradual environmental gradient. This paper describes the dynamics of the boundary on three transects over 17 years, 1982–98. Over the last seven years of that period there has been a significant increase in rainfall. At the same time, the boundary position has moved 1–2 km on the transects, with the more mesic adapted Ap. hydrosauri advancing into the distribution of the more xeric adapted Amb. limbatum. Also over the same time the density of ticks on lizards in regions flanking the boundary zone has increased for Ap. hydrosauri and decreased for Amb. limbatum. These results suggest that the environmental gradient has been altered, perhaps by increased rainfall, to favour Ap. hydrosauri, which has been able to colonize more successfully across the density troughs and extend its distribution.