Genevieve Birdsall, 18, paid to attend the concert along with her friends, and found out about the free admission from a security guard.
"It's kind of annoying, seeing as we paid," she said.
Frustrated fans also took to Facebook to vent their disappointment.
"What the f*** thats bulls*** i said [sic] 170 for my ticket", wrote Grace Asher.
Renee Bryson wrote, "That f'n sucks if for real. Paid good money to come and could get in for free cause only wanted to see evening stuff anyway. Really bad form BDO."
"I'm really annoyed with the freebie entry. I paid for 2 tickets, didn't get to go coz of a massive migraine, and now discover that I really shouldn't have bothered buying them as pple were let in for free anyway!!", wrote Fiona Louise.
However, long-time BDO fan Phoebe Mason, 20, was delighted about the parting gift from organisers.
She heard about the deal from a friend just after 6pm and the two drove to Mt Smart Stadium, experiencing no traffic and finding a car park easily.
"We had both been quite a few times; this was my fifth [Big Day Out] so I had quite a few tickets.
"It was like the end of an era and I wanted to be a part of it," she said.
Ms Mason, who arrived around 7:30, said the crowd ballooned in size and,despite smaller numbers, the event had the atmosphere of previous years.
"I can imagine if I had paid and then heard people were getting infree I would be kind of annoyed.
"I think they made the right call. If I had paid I wouldn't have been disappointed by the show."
Mason and her friend bought food and merchandise in return for the organisers' generous offer.
Speaking with TV3 during the Big Day Out on Friday, Kiwi festival promoter Campbell Smith estimated the crowd numbered around 20,000, compared to 45,000 in previous years.
