Two teenagers have been charged with planning an attack using guns and explosives on a Australian school. Picture: Reuters/Ricardo Moraes

Sydney - Two teenagers have been charged with planning an attack using guns and explosives on a Australian school and police said Wednesday they intended to kill multiple people. 

The plot targeting the school in South Australia's fruit-producing Riverland region, northeast of Adelaide, came to the attention of authorities earlier this month and the two young men were arrested soon afterwards.

Detective Superintendent Des Bray said they were accused of planning "an attack at a school involving the use of firearms and explosives which could have led to multiple deaths".

"There is no doubt in my mind that we prevented a catastrophe," he said.

"We believe most likely the attack would have occurred, if not stopped, before the end of the current school term (in mid-December)."

The two boys – aged 16 and 18 – are in custody, with the younger one charged with a count of solicit to murder, while the older one faces threatening to kill offences.

The Adelaide Advertiser cited police as saying "prohibited weapons, homemade body armour and materials that could be used to make bombs" were found.

The alleged plot was foiled a day after police in Melbourne arrested a 20-year-old man. They said he was planning to buy a gun and kill as many revellers as possible on New Year's Eve in the city.

Police said that man, Ali Ali, was an Islamic State sympathiser, but there was no mention of any extremist group link to the teenagers in South Australia.

Bray said it would be alleged the pair were involved in planning the attack between June 1 and November 7 this year.