Forget about flying high, Air New Zealand passengers will be laid low by new seats that allow them to lie back, stretch out and sleep - a world first for economy-class travel.

While Air New Zealand is remaining tight-lipped about the new design, it indicates it has "created the ability for passengers to lie down comfortably".

It is preparing for the unveiling, on Tuesday, of a new lie-flat seat that has been a year in development. The airline has built a full-size replica of a Boeing 777-300 economy-class cabin in a shed on Auckland's Viaduct Harbour, near the Air New Zealand headquarters.

"A team of Air New Zealanders has been conceptualising, building, testing and refining a range of new products and interiors that will quite simply redefine long-haul travel and set a benchmark for competitors to follow," said chief executive Rob Fyfe.

An ergonomic furniture designer, who did not want to be named, said it would be intriguing to see what Air New Zealand unveiled.

"There would be an insurmountable problems getting the numbers in," he said. "It sounds like they could be going to stack people up on the sides, but this could also pose problems of safety."

In a previous long-haul overhaul Air New Zealand concentrated on the lie-flat business class seats.