An Afghan man who helped organise an Australia-bound boat trip on which 90 asylum seekers drowned was sentenced Wednesday to six years' jail in Indonesia for people-smuggling.

Prosecutors had earlier this month sought a seven-year term for 20-year-old Dawood Amiri, alias Irfan, who was described by the judge as a middle man in the smuggling operation.

"Dawood Amiri was found legally and convincingly guilty of committing people smuggling crimes," chief judge Mariana, who goes by one name, told the court.

"The defendant's actions caused death," Mariana said, adding that Amiri had been a "middle man" in a larger syndicate, "not an intellectual actor".

Rescuers managed to save 110 of around 200 asylum seekers, from Iran, Pakistan and Afghanistan, who were onboard when the boat sank on June 21 in the Indian Ocean off the remote Australian territory of Christmas Island -- closer to Indonesian Java than mainland Australia.

Prosecutors had said during the trial that Amiri "knew that the ship had exceeded its normal capacity but still forced the boat to leave for Australia".

While Amiri was only tried for the June 21 incident, police said he had confessed to successfully organising three other boat journeys.

Prosecutors said Amiri had an indirect link with Afghan-born people-smuggling kingpin Sayed Abbas, who is in detention in Indonesia and is wanted by Australian authorities through extradition.

Irfan said he collected around $1 million from the 200 asylum seekers, who paid as much as $5,500 each to be on the doomed boat.

People-smuggling has been considered a crime in Indonesia since 2011 and carries a jail sentence of between five and 15 years under the country's immigration law.

Canberra is struggling to deal with a steady influx of asylum seekers arriving in Australia by boat, many of whom use Indonesia as a transit hub, boarding leaky wooden vessels after fleeing states such as Afghanistan and Iran.

Although they travel in relatively small numbers by global standards, asylum-seekers are a sensitive political issue in Australia and a likely hot topic in the nation's elections in September.