When US President Donald Trump stepped over the line dividing North and South Korea at 3.46pm last Sunday after shaking hands with Kim Jong-un, the moment had been a while in the planning.

Mr Trump actually mentioned his plans to visit the sensitive demilitarised zone and the possibility of meeting Mr Kim to the Washington political newspaper The Hill six days earlier, but their story was held at the request of the White House on security grounds.

But the North Koreans would have been told well in advance. The timing of the detention of Australian student Alek Sigley on the Tuesday of that week supports the theory posed by Sydney-based Korea expert Leonid Petrov - and shared by others - that Mr Sigley's mysterious detention was related to the Trump DMZ visit.

Zealous North Korean officials, knowing that a major political moment was coming, likely wanted to ensure the social media silence of a prolific western commentator like Mr Sigley, even if his writings were inoffensive to the country's regime, said Dr Petrov, who taught Mr Sigley at the Australian National University and remains friends with him.