G'day all,

The 'common name' "she-oak" was applied to the casuarina, which scientists named (in Latin) for its leaves' resemblance to the feathers of the cassowary ... a large, flightless bird of Australian and New Guinea.

Its timber has a grain resembling that of oak ... but it was considered of lesser strength ... so they added the prefix "she-" ... to indicate 'weaker / inferior' (well, they were a bunch of blokes ... !).

Anyway, the Australian National Dictionary (an Oxford University Press publication with full OED historical treatment of all distinctively Australian words, terms and usages) goes on to:
2. transf. Beer brewed in Australia.

A nautical usage (well ... at least in Australian ports ...) is:

3. Special Comb. she-oak net, a safety net slung under the gangway of a ship (see quots).

This is related back to the beer ... not the timber:

"1934 T. Wood Cobbers 163 The 'She-oak Net' … is to catch a man if he slips when going aboard; and the name … is in memory of She-oak beer, which used to be, and may be still, a powerful agent in making him slip."

All or none of this may be related to enigmatic reference in the song!

Regard(les)s,

Bob