Because I remember at some point wanting to find out if death cap mushrooms grow in Gippsland, and finding out that they do not.
IMO this is clearly another of her inferences, intended to subtly confirm her innocence. I mean how could she have intentionally foraged for and intentionally used deathcaps, she thought they grew nowhere local right?
But Death Cap mushrooms do indeed grow in Gippsland.
As any born and bred Victorian would know, I'm sure. Especially one with an interest in mushrooms.