There have been 28 cases of the mbandaka strain of the virus reported in New Zealand since the start of the year.

The Ministry's Director of Public Health says this type of salmonella bacteria had not previously been seen in the country since 2000.

Dr Mark Jacobs says most of the recent cases have been in the South Island, and 10 were in the Nelson/Malborough region. The elderly woman died in Nelson Hospital.

He says the infected people have all had a virus with the same "fingerprint".

Dr Jacobs says the most likely source is a type of food, but ministry and ESR tests have so far failed to track it down.