Interesting, after Googling, it looks like both Weetabix and Weetbix are a thing. It was started in Australia as Weetbix and eventually expanded internationally with a factory in England. They renamed it Weetabix to differentiate the product from that sold in Australia, New Zealand, and South Africa.

An interesting internet rabbit hole I didn't expect to go down today, ha! My English grandparents often ate Weetabix, and when you said it was Australian, I didn't believe you (and had a typo lol.) Turns out it's both. I'd assumed Brits must have introduced it over there, but it's the opposite.