I’m not sure that’s entirely true. All Victorian regionals (including Prime) switched from relaying Seven News to National Nine News in 1987, five years before aggregation. Prime switched back a year later when HSV returned to a half-hour news.
I’m not sure that Seven was a dominant program source, either. When you consider that the regionals pretty much all took up Nine shows like Midday, ACA, Sale Of The Century, 60 Minutes, Hey Hey It’s Saturday, Sunday plus WWOS, cricket, Days Of Our Lives… that’s a lot of airtime of Nine content, plus whatever movies or US shows it ran from Nine.
Certainly Nine didn’t have a programming monopoly but for example at the time that Southern Cross was hoping to get the Nine affiliation in Victoria, it was reported that around 55 per cent of its schedule was based on Nine-sourced programming, according to one of the trade magazines (I think it was AdNews). So when VIC TV scored Nine and SCN had to go with Ten, it meant SCN losing a large chunk of programming that they’d been showing for years. Possibly evidenced by the fact that from day one of aggregation, VIC TV pretty much dominated the ex-Southern Cross markets.