I'm also only an undergrad-trained non-geologist but I do work amongst real geologists. However, without a bit more detail I doubt they can help. A few things that could help:
- a bit more detail about the location
- if it was on/near an old road, is it local rock or was it brought in as part of the road building
- a photo of a fresh face, with some sort of scale (eg coin, tape measure)
- how hard is it
- can you test it for streak
- as said before, do an acid test
Being very very light argues for either a volcanic rock that contained significant amounts of gas or maybe a plutonic rock where crystals of a reactive mineral have weathered out from a less reactive mass. I suppose limestone or soil carbonate are possible but the weathering doesn't look right to me. Perhaps the best match I've found in a quick search is a
trachyte, which do occur is SE Qld.
Get back to us with more info; these little mysteries are always fun.