It's common for dialogue in movies to be re-dubbed in post-production, to ensure the voices are as clear as possible. Usually the actors re-record their own lines, but it's possible that the actress who played Heather wasn't available.

Well it does seem pretty safe to assume the Locket story takes place in America so it wouldn't make sense to have an Australian accented woman speaking for the part of an American girlof course that in mind, they didn't get someone to dub Al Pacino's lines in Scarface, but I suppose it would be harder to teach an Australian to speak with an American accent than it is to teach an Italian American a Cuban accent. Or perhaps they were pressed for time when making the movie and that was the quickest solution.

I know the girl who played Heather was on MTV's The Real World (A really long time ago; I think it was the third or fourth cycle). She has a pretty thick Australian accent (or did, anyways). Maybe they thought it didn't fit well with the story setting/character?

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