From Feb 1994.
A very early(?) in-program promo graphic for Heartbreak High:
The “That’s [Insert City Name]” jingle sounds like it was based off ABC America’s “Something’s Happening” promo for 1987-88. Obviously the Sydney one from 1994 was re-jigged but it’s more evident in the “That’s Melbourne” promo. Also the way the promos were shot and feature ordinary people in addition to the network personalities are similar.
ABC America Promo 1987
Channel 10 Melbourne Promo
I assume that would have to be one of the first promos for Seinfeld on Channel 10 given it was on Channel Nine the year before.