The Institute of Environmental Science and Research (ESR) said flu could peak as early as June, and most common winter illnesses were on the rise.
Emergency department doctor Kate Allan said her colleagues were starting to see more respiratory patients.
"The system is under incredible pressure and we are aware that winter brings additional pressure with that, especially in the paediatric age group."
Emergency departments (EDs) needed support from decision makers but had yet to see a winter plan, she said.
Te Whatu Ora/Health NZ's clinical lead Richard Sullivan said there was a plan - it had just not been released in a single document yet.
It centred around preventing hospitals getting clogged, so people could be admitted when needed.
That would likely mean some people waiting for non-urgent surgery would have their cases delayed.
"You'll see different pressures in different hospitals," he said.
"Some hospitals will be able to continue their planned care... others might get a rush of a certain flu in the community, which might put pressure on for a week or two so they might have to adjust it back, but then obviously they can pick it back up."
Other measures included hospital patients being monitored and cared for at home when they were well enough, and a push for timely care in the community to prevent the need to come into emergency departments.
There was a team working all winter to predict what might happen and to possibly spread the load between hospitals, he said.
"So we can, days or weeks out, say, 'Well this is what we think is going to happen based on what we're seeing around presentations, so we can adjust those levers as we need to'."
Allan, who is the NZ chair of the Australasian College for Emergency Medicine, said her colleagues were ready to go this winter.
They wanted people to be able to come to EDs when they needed to, and for there to be space in the wards when they needed to be admitted.
It would be reassuring to see a written plan, she said.
ESR national flu centre director Sue Huang said influenza used to peak in July and August in New Zealand, but for the past two winters, it had peaked in late June.
It was very difficult to predict what direction it would take this year, but at the moment there were two main strains circulating - H3N2 and H1N1.
In 2022, H3N2 dominated, with short sharp peak, she said.
"And then last year we had H1N1 virus and the Flu B both circulating, it was like a slow burner from the beginning all the way to the end of the flu season."
She urged people to get vaccinated, especially if they were vulnerable to serious symptoms.
Covid-19 was also being picked up in surveillance and was likely to start rising again, she said.
