"This is an exciting solution for passengers trying to get on buses full to capacity in the morning and afternoon peaks," Waikato regional councillor Russ Rimmington said.
"Each bus has 87 seats and the ability to carry 13 standing passengers on the bottom level. All up that's 100 people and almost 20 more than our next biggest bus," he said.
"In the city we now have a completely refurbished or new bus fleet. All buses are fully accessible and air conditioned, and the next step is free on board Wi-Fi on the Hamilton bus network soon."
Go Bus commercial director Craig Worth said: "These are the first double deck city buses in the Go Bus fleet in New Zealand and we're thrilled to have them rolling out.
"Due to the height of the bus they're a little different for our staff to drive. So there's been some special training drivers have undertaken," Mr Worth said.
Buses on the Orbiter route now run every 15 minutes between 6am and 6pm weekdays and around half hourly at other times.