Spring-free trampolines made A$50 million last year but the Kiwi who invented them didn't make a cent from those sales, according to a story in the The Age.

Keith Alexander came up with the idea of the trampoline when he wanted to buy one for his daughter and his wife said they were too dangerous.

After trying several unsuccessful prototypes Alexander began using cantilevers or stretching rods rather than springs.

According to The Age, he worked with a trampoline seller and students from the University of Canterbury, where he was employed, to develop the concept.

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It took 15 years to perfect but the trampolines have now become a worldwide success and last year generated A$50m ($55m) in sales with 25,000 sold including 7692 sold in New Zealand and Australia.

But he doesn't own any of the inventions as he signed over the rights to it to the university a few months after he started working there.

Canadian investor Stephen Holmes bought the prototype paying a royalty fee to the university and then sought to buy it outright.

Alexander got a share of the fee which was about A$100,000, The Age reported.