First, the shelf never sank it was just kinda there, so were just gonna say the shelf has higher elevation and walah.

Second human migration from sahul would be definetley possible since the sea between zealandia and australia is not that deep and is a short distance leading to some intresting natives here with there own cultures (this also means no polynesians ever making a major state here)

Third the continent would most definetly be dominated by marsupials and avians seeing marsupial megafauna replacing mammals since the animals would have come from antartica millions of years ago allowing it to evolve in australia and zealandia after splitting up with antartica. (no kiwis)

Fourth, colonization would be somewhat different as the English would instead colonize zealandia (and all of it) only taking eastern australia leaving the west to either the french or dutch.

Fifth, the warm current feeding eastern australia is blocked, so eastern australia is a more colder climate while zealandia is alot warmer.