The elderly patients come from New South Wales (six), Victoria (one) and Queensland (three).
Onset of illness notification ranges between 17 January and 9 February and all cases consumed rockmelon (also known as cantaloupe) prior to illness.
The outbreak includes three deaths, according to PMA Australia-New Zealand.
Link to grower and production stopped
NSW Food Authority is advising consumers vulnerable to Listeria infection to avoid eating rockmelon.
The agency said overseas evidence suggests contaminated water, fertiliser, contact with pests/animals or insufficient cleaning of rockmelons prior to sale could be contributing factors to them becoming contaminated.
The outbreak has been linked to a grower in Nericon, New South Wales.
The company ceased production on 23 February after being notified of a potential link between its products and illness and is investigating with the authority to find out how contamination could have occurred before resuming production.
PMA-NZ said Listeria monocytogenes has been found on rockmelons from a NSW supplier.
Any affected product is being removed from the supply chain so rockmelons available on shelves are not implicated in the outbreak.
Food Standards Australia New Zealand (FSANZ) is coordinating the trade recall of whole rockmelons.
Consumers are advised to throw away any of the fruit they have at home.
Listeria in rough surface of the fruit
A Salmonella outbreak in the country was also linked to rockmelons in 2016.
Older people and those with weakened immune systems due to illness or pregnancy are most vulnerable to Listeria infection.
Fresh produce such as rockmelons are grown in the same environment as Listeria is present, said PMA-NZ.
Environmental conditions mean the pathogen has difficulty multiplying to large numbers in the field but does persist.
The rough surface of the rind (skin) of the fruit provides a place for Listeria to survive, particularly if it is injured due to rain and irrigation water splash.
The six NSW cases linked to rockmelon form only a portion of the increased listeriosis infections in the region this year with 15 people affected, mainly elderly with underlying health conditions.
NSW Food Authority and NSW Health are continuing investigations into other sources of the remaining cases.