For years it was French River, Parry Sound,  and east to Mattawa. ( I worked for Ontario Gov for 26 yrs) then the Muskoka cottage owners from Toronto saw grant and tax advantages , and convinced some Governments to change the line . A travesty considering how short changed the real Northern Ontario is.  So it seems what used to be " Central Ontario"  no longer exists  .....don't mind me just venting ! But Toronto's backyard should not be considered " Northern Ontario" ....no wonder many in North Western Ontario would gladly join Manitoba, to get better services and highways. 

Use to be near North in-between.  

The extended federal and provincial quasi-administrative regions of Northern Ontario have their own boundaries even further south in the transitional area that vary according to their respective government policies and requirements. Ontario government departments and agencies such as the Growth Plan for Northern Ontario and the Northern Ontario Heritage Fund Corporation define Northern Ontario as all areas north of, and including, the districts of Parry Sound and Nipissing for political purposes,[1][2] and the federal but not the provincial government also includes the district of Muskoka.

Northern Ontario is a primary geographic and quasi-administrative region of the Canadian province of Ontario, the other primary region being Southern Ontario. Most of the core geographic region is located on part of the Superior Geological Province of the Canadian Shield, a vast rocky plateau located mainly north of Lake Huron(including Georgian Bay), the French River, Lake Nipissing, and the Mattawa River. The statistical region extends south of the Mattawa River to include all of the District of Nipissing. The southern section of this district lies on part of the Grenville Geological Province of the Shield which occupies the transitional area between Northern and Southern Ontario.

Northern Ontario is typically defined as French river north.