Jim Anderton's new political vehicle has gained enough members to register as a political party.
Progressive Coalition general secretary Marty Braithwaite said a mailout to 3500 potential supporters late last week had resulted in more than 500 new memberships.
This is the number required for a party to register with the ElectoralCommission.
The target audience of the mailout had been "known supporters, members of the Democrat party and a number of people who have been Alliance members".
"We wanted more than 500 by [Tuesday] in order to register, and we have more than achieved that."
The registration paperwork was now being completed.
"We had expected the significant support would come from parts of Auckland and Wigram [Mr Anderton's constituency] . . . but we have been pleased about the support from the regions, such as Southland, central North Island and pretty much all over."
Mr Anderton and his supporting MPs were expelled from the Alliance after he decided to set up a new party in response to some members' criticism of Government policy.
Opposition MPs have accused Mr Anderton and his supporters of deceit, because he retained the title of parliamentary leader of the Alliance to avoid being forced to resign from Parliament.
Alliance party president Matt McCarten said the party had retained the majority of its members, but was currently going through the lists to see "who is in and who is out".
Most defectors appeared to be from the Democrat party and those living around the electorates associated with Mr Anderton and his MPs.
The Alliance was not concentrating on building membership for the sake of it, but working to increase the number of those who wanted to take an active role
"We are concentrating on the activists ... the passive members on the mailing list are not my major concern."
The Alliance and the Progressive Coalition each face a difficult task.
Mr McCarten's aim is for Alliance leader Laila Harre to take the Auckland seat of Waitakere. Mr Anderton is aiming to hold the Christchurch seat of Wigram.
Mr Anderton is facing the easier task, as he holds the seat with a comfortable majority.
Both parties are aiming to cross the 5 per cent vote threshold, which would remove the need to have an electorate seat.
Mr Anderton would also have to reach that mark to ensure his five MP supporters return to Parliament with him.
