"A Chinese citizen, when transiting at a New Zealand airport, was subjected to harassment and interrogation without cause by NZ security and intelligence agency personnel," the embassy said late Tuesday.

"His personal digital devices were confiscated, and some of them have not been returned," it added.

The embassy also listed "several reported cases of Chinese citizens experiencing similar harassment during entry or transit in New Zealand."

Beijing "deplores the unwarranted conduct and harassment", it said, demanding that "such wrongful acts cease immediately".

The embassy has issued a complaint to New Zealand's foreign ministry and "reserves the right to make further responses", it warned.

Wellington in response insisted all people entering the country had to abide by laws and regulations -- specifically "national security and border security".

"These laws and regulations are applied in a non-discriminatory manner regardless of country of origin," its foreign ministry said.

"These points have been relayed to the Chinese Embassy."

And New Zealand's intelligence agency said any steps it had taken would have been "in accordance with the law" and "subject to robust independent oversight".