For most motorists the humble registration plate is an afterthought, only thought of when the time comes to renew or replace it, but for others it can be a prized possession often worth more than their own car.

At an auction in Melbourne in February many heritage plates were sold for more than the price of a luxury vehicle.

People crowded into the Shannons auction house, weighing up the items for their investment potential, sentimental worth or simply the prestige they would add to their car.

Registration plates were first issued in Australia in the early 1900s.

"They went from number one to 285,000. In 1939 those plates were discontinued," Shannons national auctions manager Christophe Boribon said.

As old plates were handed back over time, state governments stockpiled them, then reissued the plates in the mid 1980s in "great plate auctions".

However, the market for plates really took off in the past two decades, with New South Wales and Victorian plates the most sought-after, and some reaching hundreds of thousands of dollars.

"A lot of people look at it as a piece of art in a way because they're made out of heavy enamel," Mr Boribon said.

Buyers motivated by prestige, investment potential

Registration plates were first issued in Australia in the early 1900s. ( ABC News: Emily Stewart )

"It's pure collectability, it's prestige," Shannons' David Johnston said.

"The lower numerically the number plate is the more desirable it is.

"There's only one that says 419, as with all the other numbers, so the lower the number you can have the better."

Some buyers are car lovers, such as one bidder who paid $18,000 for 82-911 to match his Porsche.

But others see the registration plates as a unique investment.

"We see them purchased for super funds to be kept. They're roughly growing at 10 per cent per year," Mr Johnston said.

'It's irrational but we love them'

People crowd in to the auction house in the hope of picking up a heritage plate. ( ABC News: Emily Stewart )

Queenslander Shane Moore is an avid plate buyer.

"My first number I bought was 697 — that was one that came up at the auction at a reasonable price," he said.

"My second heritage plate was 2013. That did have some meaning. That was my daughter's year of birth and that was the same year I bought the plate."

As a hobby, he bought an online licence plate site. Only a dozen of the 8,000 plates for sale are heritage.

Mr Moore said one-digit plates were worth millions.