A traveller returning from the Cook Islands has been slapped with a $600 fine after failing to declare protected clam shells they brought back - which they had hoped to use as bathroom ornaments.

Now authorities are urging people to remember the rules about bringing back local treasures from overseas; as many parts of the world and the Pacific, in particular, start to open up again.

The person who arrived from the Cook Islands failed to declare six giant clam shells they had picked up from a beach.

The shells were of the species called Tridacna Maxima, also known as the Maxima or small giant clam, and had been brought back to be used as bathroom ornaments.

The shells, however, are threatened by international trade.

In a second case, a person arriving from a holiday in Fiji last month brought back several pieces of stony coral they wanted to use in their fish tank.

Yet all stony corals are protected by the Convention on International Trade in Endangered Species regardless of where they are from.

Both travellers were fined $600 under the Trade in Endangered Species Act 1989 for importing the items without the required permits.

Department of Conservation spokesman Clinton Turner said the two incidents reinforced the need for travelling Kiwis to be aware of the rules when bringing back treasures to remember their holidays by.

In both incidents, neither traveller fully declared they had such protected items - something that is fundamental in international travel, Turner said.

"We understand people want to bring home trinkets and tokens of their holidays. But if those items are wildlife, they need to check the rules first.

"They also need to know the contents of their luggage and be honest with border officials about what they are bringing in."

Tuner said authorities wanted to emphasise to all Kiwis planning to travel overseas soon to check the rules around importing wildlife items.

"Knowing the rules, getting the right permits and being honest at the border is a far better option than being embarrassed and given a fine when you return from your overseas break."