An onion sprouting in a handbag and a dead rat inside a boot were among the items seized at the border last month.

Biosecurity officers screened more than 500,000 passengers arriving in the country in December, Biosecurity New Zealand's northern regional commissioner Mike Inglis said.

Staff last month slapped 511 travellers with $400 fines after failing to declare items which pose a security risk, up from 467 issued in November.

Inglis called the infringement notices "a wake-up call for travellers who unintentionally expose New Zealand to biosecurity risk".

"Those who deliberately smuggle biosecurity goods face prosecution and potentially much stiffer penalties, including imprisonment."

Fresh produce was the most common undeclared item stopped at the border in December with 420 seizures.

More on this topic

'Let the cat out of the bag' - Airport X-ray reveals hidden feline

November 23, 2022

Snake on a plane: 1.2m-long boa constrictor found in luggage in US

Mon, Jan 9

Mishandled luggage at Auckland Airport leaves passengers infuriated

December 21, 2022

It was followed by used equipment such as hiking boots and camping gear with 83 seizures.

Meat products were the third-highest with 58 seizures.

"These items could contain animal or plants diseases, or destructive hitchhiker pests such as fruit fly or brown marmorated stink bug," Inglis said.

Among the more unusual items stopped at the airport were an onion sprouting in a passenger's handbag, and a dead rat detected inside a boot inside baggage.

He said international travel ramping up and the return of cruise ships have "contributed to the workload" of biosecurity staff.

It comes amid reports of lost or mishandled luggage at Auckland Airport over the Christmas period.

"We have been working with Auckland Airport, ground handlers and airlines to address the issue, and have rostered more staff to do biosecurity screening of those unaccompanied bags and we are successfully clearing that baggage."

Inglis said they are also working alongside the cruise industry to ensure ships are entering the country with clean hulls to stop marine pests from impacting our environment and marine ecosystems.

"It's important to balance the needs of visitors to New Zealand with the need to protect our special marine environment."