Green Party MPs' travels affect not only their consciences but also their wallets.
Some politicians pay $1500 a year to offset the carbon emissions of their journeys.
All 16 Green MPs have set up carbon accounts and pay to offset their flights, taxi trips and rental car travel out of their personal earnings.
Co-leader Metiria Turei said her carbon bill was the largest in the party because of regular flights to her hometown, Dunedin. She paid between $1000 and $1500 a year. This covered international travel, twice-weekly domestic flights between Dunedin and Wellington while Parliament was sitting, and all ground travel except bus trips.
Ms Turei said offsetting alone was not enough to reduce emissions, and she tried to minimise her long-haul travel as well.
"It's a constant concern for us, but we're required to do a job, and that requires air travel.
My carbon debt will never be repaid, and I certainly can't repay it with money.
"I have to repay it in my time and commitment to the green kaupapa."
The Parliamentary Service uses expense forms to tell the MPs how many kilometres they've travelled on ground or in the air.
The MPs then use carbon measuring schemes - such as CarboNZero - to work out their carbon footprints and make arrangements through various schemes to pay them off.
This usually involves paying $25 a tonne of carbon dioxide to companies that have made an emissions reduction.
CarboNZero, which is owned by Landcare, estimated that the trip by Ms Turei and fellow MP Kennedy Graham to Senegal caused around 16.22 tonnes of emissions, which meant a total bill of $425.
CarboNZero said international flights were not significantly more carbon-heavy than domestic travel, because a huge proportion of the emissions occurred during takeoff and landing.
The company calculated air travel emissions by taking into account the number of passengers, the distance travelled, and whether the travel was domestic or long-haul.
Green Party communications director Andrew Campbell said it was not mandatory for Green MPs to pay a carbon bill, but it was a "no-brainer" for them to sign up.
Most MPs were paid between $140,000 and $180,000 a year, so a carbon bill of $1500 was around 1 per cent of their income.
The Greens have approached Parliamentary Services to encourage an offsetting scheme for all MPs, but it hasn't been adopted. The National and Labour whips didn't know whether their MPs offset their emissions.
Bus trips and other public transport were not included in the MPs' carbon footprints, partly because the amount was small, but also because it was harder to measure.