The North Korean statement did not provide details about how it allegedly broke up the assassination plot or if anyone else involved was in custody.
The accusation alleged that the United States and South Korea had plotted to murder "supreme leader" Kim Jong Un and others among the North Korean leadership with a bio-chemical weapon.
North Korea has repeatedly denied any involvement in Kim's death.
Mr Turnbull seized on the "bitter resentment" North Korea had recently expressed against China.
"Yet Chinese and American analysts alike point out the likely limits of China's willingness to pressure North Korea to the extent the Trump administration desires", he said. Officials in Seoul said the statue claim was "completely false". South Korea is not willing to sacrifice itself for U.S. global domination, and that is true also for Asia, where the Philippines and other countries like Thailand are calling into question the USA strategy towards China since they have very important ties.
June 2014: A North Korean working at a timber firm in the Khabarovsk territory of far-eastern Russian Federation is "corrupted and bribed" by CIA and South Korean intelligence agents. North Korea has repeatedly demanded their return.
At barbecue restaurant On The Grill in the centrally located Myeongdong shopping belt, customers are "eating and partying, complaining about work and their bosses", said Singaporean owner Johnathan Quek, 30.
The article alleged that a North Korean named "Kim" had accepted money to become "a terrorist full of repugnance and revenge against the supreme leadership of the DPRK".
The U.S. has sent a warship to the region and installed a controversial anti-missile defence system in South Korea.
The US Embassy in Seoul and South Korea's National Intelligence Service were not immediately available for comment. North Korea often uses colourful language and is no stranger to extravagant claims.
Unwilling to acknowledge that its people are fleeing, the North regularly accuses the South of abducting North Korean citizens.
The North Korean statement said the plan had "been put into the extremely serious phase of implementation".
North Korea said it would attack the news companies if South Korea didn't apologize for orchestrating the "vicious smear campaign", although it didn't carry out the threat.
In a potential sign of escalating tensions, North Korea has accused the United States and South Korea of having conspired to assassinate its leader, Kim Jong-un, with "biochemical agents".
Photos from the North's state media showed Kim laughing as his special forces troops attacked what appeared to be a full-scale mock-up of the Blue House before setting it on fire.
Tuesday's vote is being held early because the former president, Park Geun-hye, was ousted and jailed over a corruption and influence-peddling scandal.
North Korea warned this week that U.S. hostility had brought the region to the brink of nuclear war.
"The price that Beijing pays for its longstanding support for a North Korean regime that exports instability as its only sure means of regime survival is a bill that has finally come due".