If Mrs Prime resigns as councillor before this Thursday, nominations to replace her will be open from September 27 to October 25.
Voting packs will be posted out from November 29 with the last votes accepted and the result announced on December 21.
If, however, Mrs Prime resigns on or after this Thursday, nominations will be open from November 24 to December 22, with voting from January 26 to February 17.
So far Peter Gill and Jane Johnston have said they intend to contest the byelection.
Mr Gill, a motoring writer from Waipapa, was John Carter's sole challenger in the 2016 mayoral election, winning 4485 votes to Mr Carter's 10,889.
Ms Johnston, of Haruru Falls, founded the Kerikeri Ratepayers Association and is the secretary of the Paihia District Residents and Ratepayers Association.
She stood in the general election as the NZ First candidate for Upper Harbour, a seat held by deputy Prime Minister Paula Bennett, and was number 47 on the party list.
The byelection will be conducted by Auckland-based Election Services with the cost determined by the number of voters in the ward.
The Bay of Islands-Whangaroa ward, where Mrs Prime held one of four seats, is the largest in the Far North with about 19,200 electors.