The election of two Super City politicians - New Zealand First's Tracey Martin and the Greens' Denise Roche - to Parliament will result in one, possibly two, byelections.
Ms Roche said she would resign from the Waiheke Local Board, and Ms Martin expected to decide within 10 days whether to stay or resign from the Rodney Local Board.
Ms Martin said there was no legal requirement to resign but she was considering whether she could combine representing her local community and being an MP.
Said Ms Roche: "I have said that I don't believe I can serve my community doing both roles and I will resign."
Council officers have put the cost of a byelection at about $38,000 in Rodney and $28,000 in Waiheke. It could cost less to hold two byelections at the same time.
They must be held within 82 days of a resignation. Possible dates include March 7 and April 4.
Ms Roche and Ms Martin have followed Jami-Lee Ross in making the move from the Super City to Parliament.
Mr Ross resigned his Auckland Council seat in March after being selected as the National Party candidate for the Botany byelection, which he won.
A strong campaigner against wasting ratepayers' money, Mr Ross defended the $150,000 cost of a byelection in the Howick ward only three months after he started work as an Auckland councillor.