The family said in a statement that John Short, 75, was arrested in Pyongyang on Sunday, the New York Times reported.
His wife, Karen, said the trip was her husband's second to North Korea. The Shorts live in Hong Kong and own and operate a Christian bookstore.
The detention of Short, from Barmera in southern Australia, comes more than a year after North Korea arrested Kenneth Bae, an American missionary, after he entered the country from China. Bae was sentenced to 15 years of hard labor for committing "hostile acts" against the North, and Pyongyang has resisted strong American pressure to release Bae, who is in ill health.
In an interview with That's Online, a website devoted to Chinese affairs, Karen Short said Australia's Department of Foreign Affairs told her that her husband's situation was "a different and difficult case."
Australia and the United States do not have diplomatic relations with North Korea.
"We're Christians, and we've been here [in Hong Kong] for 40 years," Karen Short said. "He's a front-line man -- this is what he does. But North Korea is very different -- that's why his heart was to go there. I'm asking people to pray for him."
A U.N. report released this week criticized North Korea on a number of fronts, including its intolerance of religious freedom. The report said North Korea commits crimes against humanity, including "murder, enslavement, torture, imprisonment, rape [and] forced abortions."
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