I'm always extremely skeptical when someone suggests that oral traditions could possibly have survived for long periods of time. That's twelve hundred generations! Given what we know about the speed at which Chinese Whispers tend to drift, the idea that any story can last 1200 generations is an extraordinary claim that demands extraordinary proof.

Hundreds of Aboriginal tribes had hundreds of contradictory myths on hundreds of subjects. Most of them are verifiably false (e.g. those rocks over there are not, in fact, the remains of three young women who got turned into stone for disobeying their father). Some of them are probably correct by complete accident.

As for gigantic animals... well, they're a pretty common motif in myths and stories all around the world. Maybe it's some distant memory our cultures all have of the larger animals that used to live around here... or maybe it's just because giant animals are cool. In Aboriginal myths, giant animals are often really really giant, because they're used as explanations for geographic features -- e.g. rivers are caused by the meanderings of a giant snake.