This is a feature of some Australian accents.
Australian accents can be loosely grouped into three types "cultivated" (which is close to British) "general" (used by the majority of Australians) and "broad" (used by, for example, Paul Hogan, and considered to be a marker of social class)
In the broad accent the "ei" diphthong (in day) can be articulated as [ɐ̟ːɪ] or [a̠ːɪ], whereas the "ai" diphthong (in die) is [ɒːɪ̞]. These are similar, especially if you are used to the "cultivated" accent in which the "ai" is [a̠ɪ̞].
The articulation of "ei" and "ai" in some British accents is also similar, but as in Australia, it is seen as a marker of social class, and sometimes considered to be a bad pronunciation.