Passengers are still putting electronic items powered by lithium ion batteries in luggage that goes into aircraft holds instead of their hand luggage.

The warning comes from the head of the Aviation Security Service (AVSEC) in New Zealand Karen Urwin who says airport authorities are now confiscating many items found in luggage on its way into aircraft holds.

She said the risk posed by spare lithium batteries - power banks in particular - in checked-in luggage was causing the greatest anxiety

And she has put things in a clear way so people can understand the dangers.

She said people needed to remember that something that was dangerous on a plane was not necessarily dangerous in a house.

"Your plane is not like a house,” she said. “It has a pressurised cabin, it has recirculated air, it's flying at 30,000 feet."

Ms Urwin said all products containing lithium batteries, including laptops and mobile phones, should be carried into the cabin where crew could respond immediately if they caught fire.

But she said that despite the well publicised problems, people still insisted on putting lithium batteries in their checked-in luggage.

She estimated the number of power banks confiscated by airport officials numbered in the tens of thousands.

"We have enough power banks to keep the national-grid afloat for a year," she said.

"If you're packing electronic equipment make sure the battery is inside the device. Don't have a spare battery rolling around in your bag.

"This is almost the reverse of the liquids, aerosols and gels. This is stuff you need to have on you in your carry on, not in your checked baggage. Because we screen-check baggage and we will open the bag and remove it."

Karen said there were several cases overseas where mobile phones had fallen down a seat on a plane, overheated and caught fire. Or the battery was damaged by the mechanism of the chair.

The dangers of lithium batteries are well known. In September 2016, Samsung was forced to recall its Galaxy Note 7 phones after there were cases of them exploding in people's pockets.

They are now prohibited from being carried anywhere on an aircraft.

The US aviation safety regulator, the FAA, reports that in the United States there is now on average one such incident every 10 days. 

AvSax fire containment bags are now on board several major airlines worldwide and are used to deal with burning electronic devices ranging from laptops and mobile phones to e-cigarettes. 

They were deployed on aircraft 20 times in 2017. 

If an electronic device starts to seriously overheat or emit smoke the cabin crew will pour at least two litres of water into an AvSax and then drop the burning device into the bag, adding additional water as required. The water activates the polymer gel inside the bag causing it to expand around the device. Should the device keep on venting then the AvSax is tough enough to absorb the force. 

The AvSax cools the batteries in the device, reducing the likelihood of the battery catching fire but if it does go into what is known as thermal runaway when all the battery cells catch fire at incredibly hot temperatures it is all contained within the bag.

Amazingly, the water is absorbed into the internal lining of the bag so the device is dry when it is removed.