|a port in SE Australia, in E New South Wales near the mouth of the Hunter River: important industrial centre, with extensive steel, metalworking, engineering, shipbuilding, and chemical industries. It suffered Australia's first recorded fatal earthquake, in 1989. Pop: 279 975 (2001)|
To do something that is obviously superfluous; Newcastle is a city in northeast England where coal is mined: “Karen wanted to give Dad a magazine subscription for his birthday, but I said that would be like carrying coals to Newcastle, since he already has fifteen or twenty subscriptions.”
carry coals to Newcastle
Do or bring something superfluous or unnecessary, as in Running the sprinkler while it's raining, that's carrying coals to Newcastle. This metaphor was already well known in the mid-1500s, when Newcastle-upon-Tyne had been a major coal-mining center for 400 years. It is heard less often today but is not yet obsolete.