The Maori Party says Hone Harawira's new Mana Party will not make real gains for Maori because it will not be able to work with the major parties.
Mr Harawira resigned from Parliament this week to form the Mana Party and forcing a by-election in his Te Tai Tokerau electorate on 25 June.
If he wins, Mr Harawira has ruled out working with National and Labour if Phil Goff remains as leader and says he also won't work with ACT leader Don Brash.
Maori Party MP Te Ururoa Flavell says you cannot improve Maori lives in opposition.
Mr Harawira, who left the Maori Party to become an independent MP earlier this year, says you do not need to be in Government to make an impact.
The Maori Party says it will announce its candidate for Te Tai Tokerau in about two weeks' time. The last day for nominations of candidates is 31 May.
Mr Harawira said he planned to register his new party on Thursday.