The exchange provides clients with fresh needles, barrels and syringes of various sizes, and collects used needles for safe disposal.
As well as preventing disease, Mr Greenwood said the exchange is a "safe space" where intravenous drug users can receive support without stigma or discrimination of their habit - which he said can be particularly visible in provincial areas.
"It's harder in smaller towns where everybody knows everybody else," he said.
"[Users] fear discrimination more, because it's easier to out yourself. That 'junkie' stigma is huge. People expect users to look a certain way - but, actually, a lot of our clients come in suits on their way from work."
Discrimination is also "rife" in the health system.
"People are still getting a really hard time," Mr Greenwood, who has battled his own drug habit, said.
"I went to give blood one time, and the nurse made a comment about the state of my veins. When she put the needle in my arm, she made sure it hurt."
Mr Greenwood said some clients will "stay for a cup of tea" and share their struggles with the exchange workers.
Workers can offer support by showing clients how to prevent abscesses or vein damage and how to rid substances of impurities.
They can also provide referrals to other care agencies, such as doctors, dentists, food banks and housing services.
"We're kind of like the bar men behind the counter they can talk to," Mr Greenwood said.
In addition, the exchange provides condoms, which it receives from the New Zealand AIDS foundation, free of charge.
"We go through those like nothing else," Drugs Project Wairarapa manager Brendon Olsen said.
Mr Olsen has also started up a mobile service, where he travels throughout the region distributing equipment to clients with no access to transport.
So far, he has visited clients from the Wairarapa coast, to as far north as Eketahuna and Woodville.
"It's working really well - we're hoping to bring it up to two days a week."
Mr Olsen said the focus of the exchange is not to discourage drug use but ensure safety and "keep the community clean and hygienic".
"We neither condemn nor condone drugs," he said.
"You can't stop people doing want they want. But we want to keep our clients as safe as possible.
"We don't judge."
For more information, please contact Drugs Project Wairarapa on 06 370 8259, or email brendon@drugsproject.co.nz.