The Sydney Morning Herald and The Australian both reported the breach, with the former saying that the director-general of the Australian Signals Directorate, Mike Burgess, and the head of the Australian Cyber Security Centre, Alastair MacGibbon, were both on the scene.
The passwords of all users of the network were changed as a precaution.
In 2011, the network was breached by attackers who were said to be from China and its defences were upgraded thereafter.
“The Department of Parliamentary Services and relevant agencies are working jointly to take the necessary steps to investigate the incident, while our immediate focus has been on security of the network and protecting data and users,”they said.
“There is no evidence that any data has been accessed or taken at this time. However this will remain subject to ongoing investigation.
“Similarly, we have no evidence that this is an attempt to influence the outcome of parliamentary processes or to disrupt or influence electoral or political processes.
“The Department of Parliamentary Services and relevant agencies are working jointly to take the necessary steps to investigate the incident, while our immediate focus has been on securing the network and protecting data and users.
“While there is no guaranteed approach to cyber security, best practice is the ability to detect and remediate threats quickly. The department has done this working jointly with expert agencies.”
An ASD spokesperson said in a statement: "We can confirm the Australian Signals Directorate is working with the Department of Parliamentary Services on this matter to ensure the network and its users are protected. The Department of Parliamentary Services responded immediately to the detection.
"ASD and its Australian Cyber Security Centre will continue to work with DPS to understand the full extent of this network compromise.
"Meanwhile, the necessary steps are being taken to mitigate the compromise and prevent any harm.
"At this early stage, our immediate focus is on securing the network and protecting its users. Proper and accurate attribution of a cyber incident takes time."
Daniel Lai, the head of ASX-listed Canberra-based cyber security firm archTIS, told iTWire: "Clearly the international rule of law has broken down. If this morning's reports are true, Australia's democracy is under attack in an unspoken cyber war with foreign entities.
"This proves once again that old antiquated security models no longer have the capacity to protect our existing systems that we rely on to safeguard our national sovereignty and values. We need to take urgent action to protect our government institutions, our economic interests and our national security.
"What is most bizarre about this incident is the fact that we have a cyber security industry in Canberra that is unparalleled in the world and yet our government is still looking to overseas multinationals for the answers.
"If they had them we would not be so compromised. The government is still trapped in cultural constraints that are not letting them engage effectively with industry to adopt innovative security solutions that would give them a competitive advantage against these adversaries. We need to do something differently. After all, the definition of madness is doing the same thing over again and expecting a different result."
Mark Sinclair, ANZ country manager of security outfit WatchGuard Technologies, was of the opinion that a sophisticated attack requiring a mature skillset was highly likely to be executed by a foreign state-based actor or a very well-funded organisation.
"The fact that it is targeting MPs is a strong indication that it is a foreign nation. Any MP can be the target of such an attack since the attacker may be able use it as a springboard to launch subsequent attacks on other MPs on the network," he said.
"If this attack was the result of weak or stolen credentials then the use of multi-factor authentication will play a big part in preventing further attacks.”
Geoff Andrews, APAC regional director of Ping Identity, said: "Although Ping Identity has no specific insights into this incident, we are aware of several highly active state-sponsored cyber actors who have successfully compromised other government entities worldwide.
"Government ministers and MPs are high-value targets for hackers. Highly sensitive information assets should, at a minimum, be protected by multi-factor authentication, including something you are (biometric), something you know, for example a password, and something you have such as a one-time code or token.
“Sophisticated protections will overlay intelligent, dynamic step-ups in authentication based on other factors such as location, time, changing IP addresses, or other behavioural characteristics."
Michael Warnock, Australia country manage of Aura Information Security, said the breach against confirmed that distance did not provide any defence for the Australian public and private sectors.
"Both individuals and their employers need to have proactive cyber defence measures in place day in day out and also ensure that they have a strong focus on educating their employees," he said.
"As with the community in general, the Australian public sector is in the sight of cyber criminals and we should all continue to take note of the ASD Essential 8 strategies to mitigate cyber security incidents.”