Thames Hospital clinical director Dr Ruth Large worked in Waikato Hospital's emergency department on Tuesday and said it was "ludicrously busy", with waits of up to four hours as staff treated 217 patients - up 50 from the same day in 2010.
She said many complaints would have been better dealt with by a GP, but many surgeries were closed because it was a public holiday.
"There seems to be a bout of gastro going around so we had people coming in with the trots [and] kids falling off monkey bars and breaking their arms," Dr Large said.
She expected Thames Hospital to be rushed off its feet at the weekend as a result of the thousands of people heading to the Coromandel.
Middlemore Hospital had a traditionally "horrendous" Boxing Day but the number of patients from Christmas Eve was on par with last year, said spokeswoman Lauren Young.
Waitemata District Health Board clinical leader of emergency medicine Dr Andrew Ewens expected staff to be treating more patients in the next few days despite emergency departments having already been busy.
On Christmas Day the hospitals were busy with people falling off scooters and kids bouncing off trampolines. One Waikato patient's Christmas Day was ruined by a fall downstairs which resulted in multiple fractures.
Waikato Hospital's emergency department this year treated 4302 more people than in 2010, with a total of 62,201 people being helped.
Auckland District Health Board spokesman Mark Fenwick said feedback from staff was that the emergency department had been busy for the festive period.
Auckland City Hospital traditionally treated more patients during summer because people were engaging in activities such as rock climbing and water skiing which could cause injuries.