The Auckland Museum is the first in the world to get a carbon-friendly stamp of approval after making big cuts in its energy use.
The museum has spent the past two years closely monitoring its electricity, water and gas rates to improve its environmental performance.
It has now been certified through the internationally recognised Certified Emissions Measurement and Reduction Scheme (CEMARS), after reducing electricity use by 6 per cent in a year and committing itself to a 10 per cent drop in carbon emissions by 2016.
The certification measures every detail of an organisation's carbon footprint, from the number of flights the staff take to the type of lawnmower used on the grounds.
The museum's carbon footprint for last year was measured at 1805.6 tonnes of carbon dioxide equivalents; 1800 tonnes of CO2 equivalent/year is about the same as 55 average NZ households.
Facilities manager John Glen said reducing energy use in an enormous, mostly stone building created complex challenges.
"It is an A-listed heritage building, so the changes we can make are limited," he said.
"And in reducing energy we also have to keep a constant temperature to preserve the artefacts. We have to keep the conservation department happy, and not overdo it."
Mr Glen said few major changes to infrastructure were required to reduce the footprint.
Instead, hundreds of small refinements to air conditioning, taps and lighting combined to cut energy use.
"It's just small housekeeping measures that have taken us up to 6 per cent."
More efficient LED lights have been installed inside, and high-tech sensors ensure most lights and taps are switched off immediately when not in use.
The museum's water consumption also dropped in the past year, and its gas use remained constant.
Its water use for 200 staff, one cafe, two shops and all visitors roughly equalled that of 12 to 14 medium households.
The museum also installed time-controlled rain sensors and moisture-sensitive controls so it could save water inside as well as outside.
When it rained the irrigation system was geared to skip a day so the grounds were not watered unnecessarily.
Interim director Sir Don McKinnon said the museum became certified because it believed that Aucklanders wanted to know that major institutions were becoming more environmentally sustainable.
"We know the museum still has a way to go, but getting this certification is an excellent milestone."
The CEMARS certification is part of the carboNZero monitoring programme which is being expanded internationally by Landcare Research. There are about 100 certified buildings in the world, 58 of them in Britain.
Other local companies with CEMARS certification include Westpac, Meridian Energy, Spicers Paper, Villa Maria, Wellington International Airport, Toyota NZ and Antarctica NZ.