Reuters said it had been told by five individuals who had "direct knowledge of the matter" that the Australian Signals Directorate had come to the conclusion in March that the Chinese Ministry of State Security was behind the attack.
The breach was made public on 8 February and the passwords of all users were changed as a precaution. The Liberal, National and Labor party networks were all said to have been infiltrated.
Since then there has been speculation that China was behind the hack, but no leaks have made this claim. Reuters is the first to quote unnamed sources as saying Beijing was the culprit.
It said the office of Prime Minister Scott Morrison refused to make any comment as did the ASD.
Contacted for comment, an ASD spokesperson said: "As a matter of long standing practice, we cannot comment further on the details of intelligence matters or operational investigations."
The Chinese Foreign Ministry told Reuters: "When investigating and determining the nature of online incidents there must be full proof of the facts, otherwise it’s just creating rumours and smearing others, pinning labels on people indiscriminately. We would like to stress that China is also a victim of Internet attacks.
“China hopes that Australia can meet China halfway, and do more to benefit mutual trust and co-operation between the two countries.”
Australia-China ties have been rocky for the last few years, after laws were passed under the former prime minister Malcolm Turnbull to prevent interference by foreign countries in Australia's internal affairs.
The ban on Chinese telecommunications equipment vendor Huawei Technologies from bidding for contracts in Australia's 5G rollout did not help the situation.
More recently, there have been allegations that a Liberal MP, Gladys Liu, was a member of several Chinese organisations with ties to Beijing. Morrison, who had just a one-seat majority in Parliament, has hit out at the Opposition over these claims, labelling those who raise such questions as racists.