According to my research, Nine aired Seinfeld at the 10:30pm Thursdays timeslot from the start, commencing from 9th September 1993. The reason Nine stopped showing the show less than 2 months later was because they had lost the rights to Ten, which had picked up the output deal with Columbia-Tristar, who distributed the show at the time. Ten then put the show in a more viewer-friendly timeslot & the rest, as they say, is history.
Nine regained the rights in 2008 when they picked up the output deal with Sony Pictures. It was later moved to the Seven Network from December 2012 when the network picked up the aforementioned output deal, although later seasons continued to air on the Nine Network into 2013.