There is rising concern over a new strain of meningococcal disease that's killed six people so far this year.

Meningococcal W cases have jumped from five in 2016, to 26 in 2018.

Public health physician Jill Sherwood told Mike Hosking things could be set to get worse.

"Particularly next hear when we get into that later part of the year which is when we tend to get most meningococcal cases."

She said while every strain is serious meningococcal W is very hard to diagnose early which makes treating it very challenging.

"All meningococcal is a serious illness but W came present somewhat differently than the other strains and this makes it hard to diagnose early and get the treatment in."

Sherwood said the increase in cases is very unusual for New Zealand.

"Between 2013 and 2017 we ranged between zero and six cases each year, so to go to 13 last year, we started to watch very carefully. It happened towards the end of the year so we have been monitoring carefully and to be now at 26 is unusual."

She said based on overseas research it is likely these cases will continue to rise.