Tourism Bay of Plenty head of marketing Kristin Dunne said very subtle things could influence the figures such as weather and where public holidays fell.
"This decrease is so marginal that it doesn't offer any trend," she said.
Anecdotally, the region had a busy school holiday period.
About 75 per cent of visitors to the region were domestic tourists, she said.
"Visitors come to see friends and family in the school holidays. We are a very handy three-to-five day quick break without the expense of leaving the country with a young family."
The decision by councils to invest significantly more into major events should increase visitor numbers in off-peak seasons, she said. "The new cycleways developments, potential progression [of] a hotel development and the potential of a second domestic airline carrier would all cause a steep change in our visitor numbers."
Nationally, guest nights were up 5.5 per cent year-on-year.
As in the previous three months, most of the rise for May was from international guest nights, which were up by 11 per cent. Domestic guest nights were up 5.2 per cent.
Motel Association of New Zealand chief executive Michael Baines said the figures reflected a strong year for tourism so far.
"We've had an awful lot of Aussies coming, and the economic recovery in Europe has brought back the English market that disappeared for a while," he said.
"There's also good and strong continual growth out of the United States and year-on-year growth from China."
Domestic tourism was also positive, he said.
"No one should underestimate the power of domestic tourists. Lower fuel prices, job retention and a more positive view on the economy are giving people the confidence and ability to explore."
The recent batch of bad weather around the country hadn't been too disruptive, he said.
"These things happen every year at different times. I'd say it's about the same or better than last year."
