The Ikaroa-Rawhiti by-election will be held on 29 June.
Speaker David Carter on Wednesday formally declared the parliamentary seat vacant following the death of Labour Party MP Parekura Horomia.
The 62-year-old died on 29 March and had held the seat from 1999.
The Labour Party says there have already been some informal discussions with potential candidates, and the Maori Party says it is under no illusions about how tough a contest it will be.
The National Party will not stand a candidate.
Prime Minister John Key says candidate nominations must be received by midday on 5 June.