A distinctive volcaniclastic rock type is frequently encountered towards the base of the Newer Volcanics basalts of Melbourne and is typically described as weathered, brecciated basalt. The geological origins and geotechnical properties of this rock type are currently not well understood. The rock is a type of breccia, with angular clasts of relatively unweathered high strength basalt set in a porous, friable matrix resembling a hard clay. The matrix has a distinctive pale yellow brown to orange colour when observed in the unweathered state in drill core intersections. Subaqueous pillow basalt structures, in the form of globular but highly fractured basalt bodies, can be recognised in field exposures, implying that this lithology results from lava flowing into water bodies present in the paleo-topography of Melbourne, in contrast with the sub-aerial depositional environment widely accepted for the majority of the Newer Volcanics Province. That is, the process of deposition of lava directly into standing bodies of water has resulted in a highly brecciated fabric due to rapid “quenching” effects. The term “hyaloclastite” is used here to describe this rock. Because no prior description of the hyaloclastite in the Melbourne region is available in the literature, the occurrence, geotechnical properties and engineering behaviour of this rock type are not well understood posing a potential hazard to infrastructure and construction projects. This paper describes the results of ongoing research undertaken to investigate the geological origins and distribution of this rock; presents the results of initial geotechnical and geological investigations into its properties; and, describes some possible construction hazards associated with the hyaloclastite.