Alek Sigley's Facebook page was shut down on Thursday afternoon. His Twitter account has not been used for two days. His parents in Australia were expecting to hear from him but haven't.

Prime Minister Scott Morrison said on Friday morning: "It is very troubling and concerning to me and I'm sure to his family as well."

The 29-year-old student was studying Korean culture and language at Kim Il-sung university in the North Korean capital after graduating from the Australian National University in Canberra.

In recent months, he had been blogging frequently about daily life in the closed country. He had also done some work for a South Korean publication, NK News, which monitors life in North Korea.

One theory is that this may have turned him into a journalist in the eyes of the North Korean regime - Western journalists are severely mistrusted in Pyongyang. If they are given a visa to visit, they are accompanied by minders through the day and corralled in a hotel on an island overnight.

READ MORE: Australian man detained in North Korea

Mr Sigley's visa was as a student but he also produced a blog which reveals him to have been very enthusiastic about his life in North Korea - he had Korean friends, went to restaurants and was clearly enjoying himself.

He got married to a Japanese woman whom he met there.

But one theory is that he may have been naive in not realising that hard-liners in the Pyongyang regime would have taken a very dim view of his unauthorised contacts with an outside world from which they were trying to isolate their own people. What he blogged might have seemed favourable to him but not to the authorities who fear outside contact.

On this theory being discussed by North Korea watchers, doing work for a South Korean publication might have been the last straw for the security department in Pyongyang - North and South Korea remain technically at war seventy years after an armistice was accepted when they fought each other to a standstill.

One of the lecturers at ANU, Dr Leonid Petrov, who counts himself a friend of Mr Sigley, said that the student was highly intelligent.

He was an assiduous student who couldn't learn enough about Korea. He would come to Dr Petrov's seminars even when they weren't on his formal curriculum.

Dr Petrov gave Mr Sigly advice when the former student was setting up a tour business in North Korea. That business appeared to be going well. Mr Sigley had advertised a tour in August.

Dr Petrov said that the student had a rare combination of academic and intellectual ability with an entrepreneurial bent.

The man now presumed to be detained had never shown any Christian belief at the ANU - and that's important because a sure-fire of being arrested and imprisoned for a long time there is to attempt to convert people or contact Christians. It seems highly unlikely that he has done this.

Nor was he that political, according to Dr Petrov.

Mr Sigley's blog from North Korea reveals that he was interested in politics but it does not indicate that he was politically committed. If anything, he did not have a fear of the North Korean security apparatus.

His interest in the region stemmed from his background in Perth.

According to his blog: "Ever since I was a child, I've had a strong interest in East Asia - my father is an Anglo-Aussie sinologist and my mother is Shanghainese (from Shanghai in China).

"I've also always been fascinated by socialism - my father, like many in the humanities and social sciences, has decidedly progressive political views, my mother spent a tumultuous youth during the Cultural Revolution, and my favorite class in high school was history, where I took a formative semester studying the Russian Revolution and early USSR," he wrote.

Pyongyang is a dangerous place to be a foreigner who steps out of line. In 2016, a visiting American student, Otto Warmbier, was arrested there after trying to steal a propaganda poster in a hotel. He was released a year later in a vegetative state and died soon after.

It is not known for certain what happened to him. There is a widespread belief among North Korea watchers that Mr Warmbier suffered some sort of injury in prison. That injury may have been inflicted by himself in despair at his 15 year sentence or by others.

Faced with an American prisoner who was not going to regain consciousness, the North Korean authorities allowed the return of Mr Warmbier to the United States in a vegetative state.

Some Westerners who are held in North Korea are released relatively quickly. Three years ago, a BBC journalist was only held and interrogated for 24 hours.

Others who have shown disrespect to the leadership or distributed Bibles are held for longer - though there is no indication that Mr Sigley has done either.

One hope is that Mr Sigley will be released soon as a political gesture of friendship when and if Kim Jong-un and President Trump meet after the current G20 summit in Japan.

On the ABC, Australian Prime Minister, Scott Morrison, said, "The expressions of support and assistance that have come from other nations I have met with while I've been here has been very welcome. We will continue to focus very sharply on that and seek to clarify what exactly has occurred and then take steps from there."

Steve Evans is a former correspondent in North and South Korea.