Taiwan's coast guard picked up the man on Sunday after his vessel collided with other boats on the Tamsui River, which flows from the self-ruled island's capital Taipei to its northern coast.

The man -- who claimed he wanted to "defect" according to local news reports -- had served as a captain in the Chinese navy, Taiwan's minister of ocean affairs said Tuesday, adding that he was among 18 purported defectors seen recently who claimed to admire Taiwan's "democratic way of life and came for freedom".

The man and his motives for entry would be investigated, Minister Kuan Bi-ling said, as China has stepped up pressures in recent years on the island it claims as part of its territory.

Beijing's Taiwan Affairs Office (TAO), which handles cross-strait issues, said Wednesday that the man's travel by boat to Taiwan was "purely his personal behaviour".

"The DPP authorities do not need to be so alarmed and engage in political manipulation," TAO spokesman Chen Binhua told reporters, referring to Taiwan's Democratic Progressive Party of President Lai Ching-te.

"When the man returns to the mainland, we will punish him in accordance with relevant regulations."

China has ramped up military and political pressures on Taiwan in recent years, including launching war games last month that it called a test of Beijing's ability to seize the island.

It also maintains a near-daily military presence around Taiwan with warships and fighter jets, and its coast guard vessels have entered and exited the island's waters this year.

Military experts say these "grey zone" tactics fall short of an outright act of war, but serve to exhaust Taiwan's military.

Taiwan is routinely on guard for spies from China, and the man's entry on Sunday had raised alarm.

The coast guard said 10 people have received disciplinary punishment over the breach.

Taiwan's National Security Bureau director Tsai Ming-yen said Wednesday during a parliamentary meeting that this could be "one of the cases of collaborating with the Chinese Communist Party's 'grey zone' operations".

The coast guard said Tuesday that a radar operator began monitoring the Chinese boat when it was about six nautical miles (11 kilometres) off the coast of Taiwan, but did not request an inspection as they thought it was a local fishing vessel.

Even after it was later verified as a non-Taiwanese boat, a coast guard unit had failed to immediately instruct patrolling personnel to intercept it until the boat collided with other vessels at a ferry terminal.