Up to March 6, an additional seven cases of listeriosis have been linked to the outbreak, including four deaths. Cases are from NSW (6), Victoria (6), Queensland (4) and Tasmania (1).

In January 2018 NSW Health commenced investigating an increase in listeriosis notifications, around four times the expected rate. Notifications were received from rural and metropolitan public health units, affecting elderly people. Most of the affected people had consumed a number of high risk foods during their incubation period. NSW Health commenced working with the NSW Food Authority and specialist microbiology laboratories to identify any linkages between the cases.

On February 19 NSW Health were notified that bacteria from two NSW cases, which had been notified on January 26 and February 1, had an identical whole genome sequence to a case notified in Victoria. This indicated that there could be a common source outbreak. At the same time, Queensland recognised that bacteria from three patients notified there shared a common characteristic with the two NSW and Victorian cases.

At this stage a number of food items were in common between the six cases, which allowed the NSW Food Authority to better target their investigation of food retail items. Over the coming week, specialist laboratory testing from four more NSW listeriosis cases demonstrated that they were also linked to the outbreak. The food history information from the ten cases highlighted that eating rockmelon before coming ill was common amongst all ten people. This then allowed the NSW Food Authority to target their investigation on rockmelon.

On February 27 samples taken by the NSW Food Authority from rockmelons from a farm in Nericon grew Listeria monocytogenes. Later that day the Victorian Department of Health advised that samples they had taken from rockmelon from the same farm had grown Listeria bacteria which shared the same unique characteristics as the bacteria from the ten people linked to the outbreak.

On February 28 rockmelons from the affected farm were recalled, and a public alert issued.