The Newcastle District was formed from the eastern section of the Home District in 1802. It consisted of lands that now make up the present counties of Durham, Northumberland, Peterborough, Victoria, Haliburton, as well as parts of Muskoka, Nipissing, and Parry Sound. The district town was originally Newcastle, but afterwards Amherst, which was renamed Cobourg.
In 1838, the northern part of the District was separated to create the Colbourne District, containing the new Peterborough County which, at that point, contained all the other northern counties listed above.
The remaining part of Newcastle District, namely the Counties of Northumberland and Durham, was abolished in 1849. The administration of the area continued under the title: the United Counties of Northumberland and Durham.
The Archives of Ontario has produced a series of maps illustrating the growth of the District system across the province. Each map ought to be expanded to its maximum size in order to benefit from the information it contains. Return to the collection using the "back" button on your browser.
The Wikipedia article on the Newcastle District states:
It is not known if this ruling applied only to the counties of Northumberland and Durham or to all groups of counties that were made into districts between 1798 and 1838 when the last districts were formed.
Ontario Districts: an Explanation
When Upper Canada was formed in 1788 it was immediately divided into four districts: Hesse, Nassau, Mecklenburg and Lunenburg. In 1792 these names were changed to Western, Home, Midland and Eastern respectively. The expansion in population of the province, and in the area settled, obliged the number of Districts to increase. By 1849 there were twenty individual districts, each with a number of counties under its jurisdiction.