Dr Nicholson, of Combined Vet Services, said she had heard "horrible" stories of the disease killing cows in southern Southland.
The disease had been "particularly bad" this season and it was an abnormal pattern, she said.
Cows ate the bacteria and, if they were under stress, the cows struggled to fight it in their guts and became sick, often dehydrated with acute diarrhoea. Death was the worst-case scenario.
Salmonella bacteria could enter a farm by an infected cow carrying it.
At a DairyNZ field day in Eastern Southland this month, farmers asked Dr Nicholson if the wet spring attracting more waterfowl to the farms could be the cause of the spread.
"We don't know. We think that maybe that birds are responsible but we also think that there's probably carrier cows as well ... we just don't know," she said.
Ministry of Primary Industries had launched a survey to investigate why some farms got it and others did not.
A vet could provide the survey to any dairy farmer who wanted to contribute, whether they had salmonella on their farm or not.
Cows could be vaccinated for salmonella and needed two jabs if they had not had the shot before.
The ideal time for the vaccination was in April or May.
"You don't want to do the second shot too close to dry off because it can make the cows feel a little bit average for a couple of days," she said.
Federated Farmers Southland vice dairy chairman Kass Rauber, of Hedgehope, said there were more than 100 cases of salmonella on dairy farms in Southland this season.
In a normal year, about a dozen farms had the disease in their herd during a season.
The disease hit cows hard, especially during calving.
Biosecurity New Zealand animal health surveillance principal adviser Jonathan Watts, who manages the animal health surveillance programme, said Southland and South Otago had been experiencing a significant outbreak of salmonella in cattle since spring last year.
Salmonella could cause significant sickness and death, impact production and welfare in cattle and cause sickness in humans.
"We would like to understand the risk factors that have led to this outbreak to help mitigate future outbreaks, and to reduce the impact of outbreaks in the future."
