The new office, on Kerikeri Rd, will open three days a week for constituency matters and be used by visiting ministers.
Mr Mitchell said looking after both Rodney and Northland would pose challenges of time and distance, but the electorates had much in common.
Asked why the closer Whangarei MP Shane Reti was not assigned to Northland, Mr Mitchell said Dr Reti was a first-term MP still learning his role. In any case he had long had an agreement with Mr Sabin to look after the southern part of the electorate which, like his own Rodney electorate, was part of the Auckland supercity and faced many of the same issues.
Electorate chairwoman Rose May said the opening represented a "rebirth" of the party in Northland.
She believed it was the first time a party had opened an office in an electorate where it had neither an electorate nor a list MP. It costs were covered by Parliamentary Services funding for list MPs.
Mrs May said the party would run a selection process next year to choose a new Northland candidate for the 2017 election.
The new office will be open 9am-1.30pm, Wednesday to Friday.