All three party leaders will be given a chance to speak to the crowd, estimated to reach between 12,000 and 15,000.
Free sexually transmitted infection and HIV testing will be available through the Burnett Centre where consistent condom use will be promoted.
The event is sponsored by the New Zealand Aids Foundation and organised by the Love Your Condom brand, and its themes include safe sex, respect and inclusion, as well as celebrating gay pride.
"Each year the event is more inclusive and reflective of the vibrant diversity in our communities," spokesman Joe Rich said.
"It is an entertaining, empowering day, and a reminder that our rainbow communities are thriving. Keeping safe while having fun is an important part of that."
He said the Big Gay Out had been a huge contributor to New Zealand's low HIV rates.
The Queen of Rarotonga, a known supporter of gay rights in the Pacific, is also expected during the day.
The Big Gay Out is the third event of 80 planned over the next three weeks as part of the Auckland Pride Festival.
Director Ta'i Paitai said the annual festival with many free events had grown in recent years, and included theatre and other culturally diverse acts.
The festival started yesterday morning with a dawn service at Okahu Bay. A gala will be held at Q Theatre tonight.
Big Gay Out
• Where: Coyle Park, Point Chev, Auckland
• When: Tomorrow noon until 7pm
• Who: Everybody from Prime Minister John Key to Rarotongan royalty
• Why: To celebrate gay pride and promote safe sex, respect and inclusion
• Transport: From the city - LYC Big Gay Out buses depart from outside Family Bar on Karangahape Rd every 20 minutes from 11.30am. The last return bus leaves Coyle Park at 8pm. Bus fares are $2 each way. For cyclists - the Pride Ride departs Queens Wharf at 11am and Western Park at 11.50am.