The Australian election wasn't particularly close. The two-party-preferred margin was 3.2%, which is the mid-range in terms of margins this century (7 federal elections since 2000, 3 have had higher margins and 3 lower).

Yes the "death tax" was an outright lie. But there were lots of factors which went into that margin.

Also it's important to clarify that (for the most part - except possibly on some WeChat groups) - this wasn't a foreign campaign. It was mostly old fashioned paid advertising by the Liberal party itself.