Having listened to lots of macho Australian English, I'm ready to say
they sometimes confuse /f/ and /th/. You see/hear it when they do that
cut with Steve Irwin and his bit with the deaf adder, and the
for-North-American-TV over-dub. We are told that the Australian death
adder is one of the most lethal snakes in the world, equal to the
green mamba.

Steve Irwin really did have his teeth when he said it. And while we
all like a macho Oz accent, doing it well takes lots of penile
testiculuitude.