New Zealand authorities have been accused of "vandalism" after they stripped the ivory key tops from an antique piano shipped into the country by its British owner.

The 123-year-old upright piano should have been exempt from strict rules aimed at cracking down on the ivory trade, because it was built before 1914.

But owner Julian Paton, an English heart disease researcher who emigrated to New Zealand with his wife and two children, was unaware he needed a special verification certificate for the family heirloom, according to New Zealand media.

"We are disappointed and horrified as a family at the bureaucracy," Mr Paton told the website, adding that they had "followed all the rules that we were told to follow".

According to the report, the conservation department said the instrument was "deemed, by authorities in the United Kingdom, to have been illegally exported from the UK and illegally imported to New Zealand".

The Herald on Sunday newspaper criticised the department's decision as "Kafkaesque red tape" which did little to enhance New Zealand's reputation.

Mr Paton's local MP David Seymour called the saga "outrageous" and said removing the ivory was "vandalism".

"I'm embarrassed as a New Zealander and as a local MP that this is how we welcome people, by confiscating their family heirlooms so their kids can't play piano," he said.

Mr Paton, who will have to fit expensive synthetic tops to the keys, intends to fight the government order that he also pay for the removal and dumping of the ivory.

New Zealand is a signatory to the UN Convention on International Trade in Endangered Species (CITES), which seeks to protect many threatened animals and plants.