Nairn, who plays Wendy Cooper on Shortland Street, said ditching the drink was a win-win for participants.
"It is such a tiny thing to do to help raise money for a really special cause," she said. "Compared to what people with cancer live with on a day to day basis, it doesn't even compare.
"These people are in a situation none of us ever want to be in. You're having a healthy month and also helping people, so it's a win-win."
Entries were tracking 35 per cent higher than the same time last year, and more than $100,000 had already been raised.
Last July, more than 4100 Kiwis helped raise upwards of $765,000 to upgrade facilities for chemotherapy patients in Auckland, Northland, Wellington and Christchurch.
A spokeswoman for the event said the theme was encouraging people to "clear their heads" and make the most of a hangover-free month.
"It's amazing what you can do when you have a clear head," she said.
Dry July
• Participants are sponsored to abstain from alcohol for the month.
• Funds go towards improving facilities for chemotherapy patients.
• Last year, more than $765,000 was raised by more than 4100 Kiwis.
• Registrations are tracking 35 per cent higher than the same time last year.
On the web: nz.dryjuly.com.