Looks like vuggy silica, the cavities perhaps representing something like calcite or another mineral.Basalt does occur with gas voids but usually they would be roundish, whilst these seem to be be very sharp and angular. Almost as if some crystals have been dissolved out of the specimen during weathering.
Is that quartz along the top?
Perhaps a location as Grubstake suggests would be helpful as the types of rocks that generally host quartz veining can differ a lot throughout Australia
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