The author, a junior doctor at the time, later acts the consultant what plant he was referring to and the consultant says "a water lily".
I think the consultant in question had a bit of a thing about people consuming large quantities of "natural" products and being surprised that this could make them quite ill and indulged in a bit of a rant.
There actually is a tree which will kill you if you hang around it for long enough (the manchineel tree), but to be fair I wouldn't expect to find it in somebody's garden.
Slightly surprised that tree isn't native to Australia - I guess though that pretty much any tree of a decent size will potentially kill you if you stand under it long enough!
