North Korea has taken the unprecedented step of writing to the Australian Parliament in a blistering letter that demonises US President Donald Trump and which offers Australia "assurances of its highest consideration".

Foreign Minister Julie Bishop, who revealed the existence of the "unprecedented" letter to Fairfax Media on Thursday, said the missive was a clear sign international diplomatic pressure and economic sanctions on North Korea over its nuclear program and repeated tests of inter-continental ballistic missiles was beginning to work.

The letter was written by the powerful Foreign Affairs Committee of the DPRK Supreme People's Assembly and sent via North Korea's embassy in Jakarta to Australia's Indonesian embassy on September 28. From there, it made its way to Ms Bishop a week later.

North Korea has repeatedly threatened the United States, Japan, South Korea, Australia and other Western allies, warning last week after Ms Bishop's visit to South Korea that Australia would face "disaster" if it continued to "follow the US in imposing military, economic and diplomatic pressure upon the DPRK".