China conducted a cyber-attack on Australia's parliament and three largest political parties before the election in May, according to multiple sources cited by the Reuters news agency.

Five sources familiar with a classified report told Reuters that the Australian Signals Directorate determined China came to this conclusion - but two said the report is being kept under wraps so as not to disrupt the relationship with Australia's biggest trading partner.  And one said there was a "very real prospect of damaging the economy" if Australia were to publicly accuse Beijing.

China is denying the report.

"When investigating and determining the nature of online incidents there must be full proof of the facts, otherwise it's just creating rumors and smearing others, pinning labels on people indiscriminately.  We would like to stress that China is also a victim of internet attacks," read a statement from China's Foreign Ministry.  "China hopes that Australia can meet China halfway, and do more to benefit mutual trust and cooperation between the two countries."

Prime Minister Scott Morrison's office wouldn't comment on the report, wouldn't say if it had brought up the issue with Beijing, and wouldn't say if holding back the report raised any issues with the United States, which might want a much harder line with China.