A Blenheim student is selling what he describes as a "working nuclear fusion reactor" on TradeMe.
Former Marlborough College pupil Samuel Lee is selling the home-made device in order to help him pay for university.
On the auction page, Mr Lee describes the device, saying that in its present state it is capable of generating "a bit of plasma" in the reaction chamber, but also says it can be used to generate neutrons using the included vial of deuterium oxide.
Neutrons can be dangerous, as they are a form of ionising radiation, and Mr Lee warns that the operation of the device is best left to someone who knows what they are doing.
"Built-in 2018 and forged from the depths of physics," Mr Lee wrote, "a mighty machine arose from a seemingly normal garage in suburban Blenheim.
"After a year of working two part-time jobs, researching, and building, the science fair project was complete! In its current state, a beautiful plasma generator stands.
"However, by adding the second isotope of hydrogen into the plasma void ... a tasty nuclear reaction will ensue!
"It's kind of an art piece but will generate neutron emissions for other activities such as making other isotopes radioactive," Mr Lee wrote.
In terms of legality, the device is not illegal.
In 1987, New Zealand passed the Nuclear Free Zone, Disarmament, and Arms Control Act, which prohibited ships propelled (or partially propelled) by nuclear power, or those armed with nuclear weapons, from coming to New Zealand.
However, the Act does not specifically outlaw land-based nuclear devices or reactors of this type.
The price on the auction is currently set at $4000.