Rynoz owner Mr Sentch and the owners of McMorrissey's on Vine St, Bacios on Bank St, and Head Office on Water St have been challenging the council over the policy in front of the authority since 2010.
Mr Sentch believes not only will it result in fewer punters, but increased costs due to bars having to hire extra security to deal with people turned away.
"I think the biggest issues are going to come at the bar door. I guess it's taken away people's freedom of choice at the end of the day," he said.
"It's a tough time and we spend a lot of money to keep these places going. We are a tourism town here, and if there's no hospitality, there's no tourism."
But Whangarei District Council group manager for district living Paul Dell believed it would have the opposite effect.
"We hope to see people coming into town earlier than they currently do, and hopefully we'll see a reduction in 'front-loading' (drinking beforehand) or 'side-locking' (drinking from vehicles parked in the CBD).
"My view is that there is an opportunity for businesses to improve their overall operation," Mr Dell said.
Council staff were liaising with hospitality industry representatives and taxi companies, as well as putting together a communications strategy to alert punters to the change.
The council, Northland police and the Northland District Health Board supported the policy, which would be re-evaluated after 15 months.
Whangarei was the first provincial town to adopt such a policy and was leading the way nationally, Mr Dell said.