According to an earlier theory, the "petrified forest" was formed when the forest of Moonah (Melaleuca lanceolata) )trees was smothered by a large sand dune. Water seeping through the sand formed a crust of sandstone on the outside of the trunks, decaying the organic matter, leaving behind petrified trunks.
(cf. www.visitvictoria.com/displayobject.cfm/objectid.000E12C1... )

In fact, this is a more complex system of multiple-phase solution
pipes and minor rhizocretions. These have been dissolved, covered,
exhumed and re-covered in a series of dune-building and erosion phases.
www.earthsci.unimelb.edu.au/~mlcupper/Chapter_11.pdf