Sarah Dalton, executive director of the Association of Salaried Medical Specialists, said the so-called "winter surge" was now "the new normal" in many hospitals.
"It's not unusual for our medical wards to be over 100 percent occupancy even outside the winter surge.
"So if we get flu at the same time as Covid-19, that seems like too many things."
Health workers had borne the brunt of the Covid-19 outbreak in China and their experience showed clear protocols and protective equipment were needed now, before the virus arrived, she said.
"I think our whole public health function in terms of epidemiology, planning, preparedness for epidemics and pandemics has been really undercooked. And the worry is the increase in funding for health is really too little, too late at this point."
A report by the New Zealand Institute of Economic Research says New Zealand needs to do more to prepare for a pandemic and not rely on travel bans.
Its co-author, health economist Sarah Hogan, said travel restrictions could at best delay the virus arriving - and in this case, that might do more harm than good.
"Delaying the virus' arrival in New Zealand now could mean that it arrives in New Zealand during our flu season. This would put extra strain on our health sector's limited resources at the worst possible time of year."
Hogan said on a global index of pandemic preparedness by John Hopkins University, New Zealand ranked 35th out of 195 countries, behind South Africa, Lithuania and Poland and significantly behind Australia, Canada and Britain.
An international expert in infectious disease, Professor David Murdoch, said there was a real possibility Covid -19 would coincide with the flu season and the government and its health and research resources should be working together in preparation for an outbreak.
It was "brave" to implement travel restrictions but only time would tell if it was the right thing to do, he said.
"This is a time when there are some pretty tough decisions to be made by public health authorities, and often there's no right or wrong. Everyone's working with the best available information, which is changing very rapidly."
Otago University epidemiologist Michael Baker said he was impressed the government had "put public health ahead of economic and political pressures".
He said the travel restrictions and voluntary isolation regime have been effective, even if it is possible Covid-19 had already breached the border.
"Obviously there will be the odd person who has arrived in New Zealand who has had this infection, but hasn't infected enough people for it to be noticed."
The Ministry of Health said it was well-prepared for Covid-19's arrival with the National Health Coordination Centre already in action and a long-standing plan to deal with an influenza pandemic.
A spokesperson said the temporary border measures, which were reviewed every 48 hours, were "the most practical short-term option to effectively mitigate the threat of Covid-19 to public health".
"This strategy allows more time to gain a deeper understanding of the virus and the best way to manage the disease, prepare to mobilise further responses in the health sector and other sectors, and reinforce public understanding of appropriate hygiene measures."
Ministry officials are working closely with their Australian counterparts and following guidance from the World Health Organisation.