What is this nut tree?
 

This small tree grows along roadsides close to the bank of the Colo
river west of Sydney.

http://www.box.net/shared/static/mm49y5789h.jpg

It has fruits containing nuts.

The internal structure of the fruit looks something like a mangosteen,
but the thin pulp has an astringent flavour.

Inside the the thin shell is a pure white nut. It tastes pleasantly
mealy, but not delicious. I spat it out in case it is poisonous.

I don't think it is a native Australian species and there are a lot of
garden escapes in this area.

What species is it?