Not sure. Oddly, I haven't seen any promos which announce the return of Mark Ferguson yet. Even last year, there were at least "Mark Ferguson Returns" liners at the end of "Special Report" promos from what I remember.
Personally I'd prefer it if Seven quietly gave Michael Usher the role of Sunday-Thursday Sydney 6pm newsreader on a permanent basis with Fergo moving to Fridays/Saturdays but of course that's very wishful thinking!
Thankfully, Seven seemed to have no problems allowing Michael Usher to read the news on the day the NSW Premier announced his resignation so they shouldn't have any problem with Usher reading tonight's news.
When Fergo returns, I'll be watching Nine for most of the next 48 weeks or so...