The hackers used several waves of 'Trojan' viruses (concealed as useful files or links, often sent via email) to gradually extend their access to the network piece by piece, experts told the news agency.
But the infiltration was only discovered in May, when the malware (damaging software) was activated on parliamentary computers.
On May 12th, the Federal Office for Protection of the Constitution (BfV) warned MPs about the attack after they noticed that some computers in the Bundestag were controlling suspicious outside servers.
They said that only 15 out of the thousands of parliamentary computers had been attacked.
But even that small degree of access allowed the hackers to gain administrative authority, which they have since used to penetrate the entire Parlacom network.
It is not yet clear exactly when the hacking attack began as many of the log files which would be needed to check are regularly deleted.
The attack appears to have stopped for now, as government computer experts have noticed no further data flowing out of the Bundestag.
But there is a chance that the hackers' software is still present and waiting to be activated on parliament machines.