Most of the Labour portfolio, with a total capital valuation of $4.9 million, is owned through Labour Party Properties Inc, which declared mortgages of $369,000 and no income in its most recent annual return, in 2011.
Labour's general secretary, Tim Barnett, said the only property directly owned by the party was its Wellington headquarters on the bottom floor of Fraser House.
The two offices of Mr Hipkins and Ms Dyson, in Lower Hutt and Christchurch respectively, were technically owned by the local electorate committees but the mortgages came "under the umbrella" of Labour and were paid off over time.
The majority of the properties owned by the incorporated society, including the offices of Mr Twyford and Mr Robertson, also came under the umbrella of Labour but were officially owned by the local committees.
"We don't actually own them in the sense that if they were sold, the money would go to the electorate organisation, not Labour itself."
A fourth category was properties in Thames, New Plymouth - part of which includes Mr Little's office - and Dunedin that were independently owned by the respective branches of the Labour Party.
Mr Barnett confirmed that the electorate offices of the five MPs owned by Labour were rented to the Parliamentary Service at a price set by a market evaluation.
"There's no direct benefit to us as the New Zealand Labour Party. The rent is paid to the local electorate committees, who in turn pay levies to the party."
Mr Twyford told the Herald that Parliamentary Service paid a monthly rent of $1959 for his Te Atatu electorate office on Great North Rd and said there was no conflict of interest in taxpayers' money being used to rent back a property owned by Labour - or individual MPs.
"These rents are set after a market valuation and I would have thought the public would be comfortable with that."
Mr Hipkins said the Parliamentary Service paid about $1800 a month for his two offices, adding that he was fortunate that low rentals allowed him to have two premises.
Mr Robertson said the rent for his office was below the market rent.
Mr Little said the Parliamentary Service paid $750 a month for his office in a converted church owned by an incorporated society associated with the local party branch.
He said he had negotiated a lower rate for the office when he entered Parliament. "It's cost-effective and a good set-up," Mr Little said.
A spokeswoman for the Parliamentary Service said the service was unable to supply information about rents paid for electorate offices, "as we do not comment on individual members' funding usage".
She confirmed that the service required a market rental evaluation "where the member has a direct or indirect interest in the property, or a relationship with the landlord".
The service pays $1500 a month towards the rent on Mr Key's electorate office, which a spokeswoman said was the same rate set by the Parliamentary Service 11 years ago when he first entered Parliament, "and well below what the current market rental would be".
Other than Mr Key, ministers Gerry Brownlee, Nick Smith and Amy Adams and Whangarei MP Phil Heatley own their electorate offices.
Dr Smith said MPs found it useful to own their electorate offices because the Parliamentary Service did not allow for leases longer than three years.
The series
Monday
The MPs who have not declared homes they hold in their superannuation schemes.
Yesterday
The MPs who own the most property.
