"The offending involved $70,700 in sales and unaccounted for fish," documents show.
The fish cannot be imported into New Zealand under the Biosecurity Act 1993 rules.
Company director John Khan pleaded guilty to two charges related to offences that happened between December 2017 and July 2018, according to documents released by the Ministry for Primary Industries.
Banned fish slips into NZ after being declared as sardines
"Khan falsely declared that the fish were Sardinella longiceps (Indian sardines) in order to receive clearance for the importation of 3500kg of the fish.
"He sold the fish from December 10 2017 to July 2018."
The 38-year-old, also known as Mustafezur Rahman Khan, received six months' community detention and 12 months supervision.
A second investigation found that the fish was not only sold online but also at a shop in South Auckland.
That investigation involved a Supavalue food store in Ōtara, where the Khan brothers run their businesses from.
As a result, Khan's brother - Moshiur Khan, also known as George Khan - and the company Khan's 2nd Generation Ltd were convicted on related charges in January this year, the MPI said.
Each were fined $10,000 and George Khan was convicted for selling Hilsa from his physical store.
MPI Director of compliance, Gary Orr, said the Ministry takes any breach of the country's biosecurity "very seriously" - especially given the fish was imported using misleading information.
"Our biosecurity rules are there for a reason," he said.
"Anyone bringing products into New Zealand has a responsibility to ensure they are not introducing things which could be potentially harmful.
"The offending was detected by the good work of a MPI biosecurity officer, who saw the fish being sold via Facebook."
