[10][11] Another possibility is that the parish was named in honour of the Duke of Newcastle, Prime Minister of Great Britain 1757–1762.

Newcastle Parish is bounded:[2][14][15] The original boundaries of Newcastle were Westmorland County on the south, a line due west from the northern tip of Portage Island on the north, and a north-south line through the mouth of Cains River on the west.

[19] The boundary with Northesk was different, starting near the old courthouse and passing through the intersection of Newcastle Boulevard and Beaverbrook Road.

In 1850 the boundary with Alnwick Parish was adjusted by running further up the Bartibog River before turning north.

[14][15][21] bold indicates an incorporated municipality or Indian reserve; italics indicate a name no longer in official use