China has hit back at suggestions it was behind a sophisticated cyber attack on Australia's political parties, warning "irresponsible" and "baseless" speculation will heighten tensions.

Prime Minister Scott Morrison revealed on Monday cyber experts believe a state actor was responsible for the attack, which also targeted the networks of federal parliament, but didn't say which country.

Security sources indicated last week China could be behind the attack, but the government's cybersecurity chief said the culprit was not yet clear.

China's foreign ministry spokesman Geng Shuang said Beijing was firmly opposed to reports blaming the superpower for the breach, accusing media of undermining China's relationship with Australia.

"One should present abundant evidence when investigating and determining the nature of a cyberspace activity, instead of making baseless speculations and firing indiscriminate shots at others," he said.

"Irresponsible reports, accusations, pressurising and sanctions will only heighten tensions and confrontation in cyberspace and poison the atmosphere for cooperation."

He called on the international community to deal with cybersecurity threats through dialogue and cooperation on the basis of mutual respect.

Liberal, Labor and National Party platforms were hacked during a breach of the Australian Parliament House network earlier this month.

There is no evidence of an attempt to interfere in Australian elections.

But the government has put measures in place to ensure the integrity of the system, with Mr Morrison instructing the Australian Cyber Security Centre to be ready to provide the parties with support.

Centre boss Alastair MacGibbon said his team had concluded it was a nation state, but were yet to work out which one.

"There are only a handful of states that can do this type of thing but it is hard for us to really come and definitively say who it is," he told Sky News on Tuesday.

"Because of the type of infrastructure used in this case and by their methodologies inside the system, they are confident it is a nation state. What we don't know is who."

Home Affairs Minister Peter Dutton said there were only a small number of states who had the capacity for the attack.

"There's no prospect of us attributing blame at this stage, that's under investigation," he told Sky.

Speculation about Chinese hacking comes as traders halt purchases of Australian coal and coking coal while clearing times through China's customs have at least doubled to 40 days or more.

The four traders told Reuters on Monday only cargoes from Australia, the biggest supplier of the fuel to the world's top consumer, were affected.

Customs clearance typically takes five to 20 days. Now it can be as much as 45 days, said the manager, asking not to be identified.