Parliament's Speaker Lockwood Smith says MPs who own their electorate offices often provide a better service to the public.
Six National MPs and one Labour MP have a financial interest in their office buildings and lease them back to Parliamentary Services.
Cabinet ministers Nick Smith, Phil Heatley and Gerry Brownlee are among the National MPs who directly or indirectly own the offices they use.
Stuart Nash is the only Labour MP to have an interest in an office but the party itself owns six properties which it leases back for its MPs.
All electorate MPs receive allowances of at least $64,260 a year for office rent and running costs.
Dr Smith says the system is fair and sensible as the rental is independently valued and there is a cap on how much can be paid.
He says MPs who purchase an office building in their electorate often get a better deal than they would if they rented.