Police are looking into whether voters from North Island centres outside of Auckland have been fraudulently enrolled to addresses in the city.
Dozens of police are involved in the super city electoral enrolment probe.
The Registrar of Electors has already removed 306 enrolments after finding people did not live at addresses in the Papatoetoe ward.
Vote transfers from Hastings, Tauranga and Hamilton are now being investigated.
Computers taken during search warrants are being examined.
Detective Inspector Mark Gutry says about 100 people have so far been interviewed as police starting to piece together the situation.