The by-election writ day will be Monday, February 23, and the deadline for candidate nominations to be received will be midday on Tuesday March 3.
The last day for the return of the writ will be Tuesday April 14.
Shortly after Mr Key announced the date of the by-election, National said it had opened nominations for candidates.
"While the Northland electorate has been held by the National Party for many years, by-elections traditionally see lower turnouts and smaller margins," said National Party president Peter Goodfellow.
"We will not be taking Northland for granted and my message to the successful candidate will be: prepare to put in the hard yards, we've got a big job ahead."
The selection process would be "robust and highly democratic", Mr Goodfellow said, with hopefuls submitting a formal application followed by a pre-selection panel interview.
"Successful applicants then go before local part members for a final selection to elect the candidate who can best represent the electorate," he said.
National's nominations would be open until midday, Thursday February 12, with final candidate selection to take place before the end of the month, Mr Goodfellow said.