New Zealand's initial response to last year's influenza epidemic was marked by a lack of planning, inefficient use of staff, and data management "chaos", an official report says.

The first cases of the flu strain arrived in New Zealand on April 25, with an Auckland high school group returning home from a trip to Mexico.

Health authorities were notified by community doctors, that day, of the first case of illness among returning Rangitoto College students. This triggered an immediate response of screening at the border, contact-tracing, and isolation and treatment of cases at home. It is now estimated 30 per cent of the population was infected.

For many it caused only the usual symptoms of flu, but for some it was a serious illness and 35 deaths were attributed to it.

Authorities hope that with many people now immune to the strain because of their exposure last year, and a high uptake of this year's flu vaccine, which includes the epidemic strain, any second outbreak this winter will be relatively mild.

The report, obtained by the Weekend Herald under the Official Information Act, is based on an evaluation by consultant Martin Dawe of the first fortnight's response to the outbreak by the Auckland Regional Public Health Service.

It draws on information provided by the service's staff and officials from health boards and the Health Ministry.

"Findings indicate that, overall, the service has much to be proud of in terms of the speed and effectiveness of the initial response," said Mr Dawe.

But the report also found "significant" deficiencies in emergency preparedness and management - many relating to the necessary speed with which the service had to mount a large and wide-ranging response to the initial outbreak.

"Efforts were made to draw on plans, but there was a general sense for many people that plans were inadequate, absent or poorly utilised," the report said.

This is despite years of intensive national and regional planning for an influenza epidemic, including a large-scale exercise.

In 2007, the ministry declared said New Zealand's planning was among the best in the world "and we are confident the country will be ready when a big one hits".

But Mr Dawe said informants in the evaluation considered the exercises were too limited and in future should be more realistic.

"Others noted that recommendations arising out of the exercises had not been actioned by [the public health service] or others and therefore became problems in the real event."

Poor use was made of volunteers from other parts of the health system at the start of the epidemic, and the data systems were inadequate.

"I was involved in data management on a weekend and it was chaos," Mr Dawe quotes an informant as saying.

He cited the example of nurses who, because of initial inadequacies in the information systems, visited people at home - needlessly, as they had not been in contact with the epidemic strain of the flu.

The public health service's clinical director, Dr Julia Peters, when asked if it had rectified identified weaknesses, said many resulted from the sheer scale of the initial response, which had involved the near-tripling of staff numbers with co-opted and volunteer health workers.

Changes had been made, including the creation of a dedicated emergency planning and response team as part of the restructuring of the service.

Despite its 12 per cent staffing cut, the service was now in a better position than last year to respond to an epidemic, Dr Peters said.