He'd uncovered a silver sixpence of King George III, dated 1817.
Born in London in 1738, George William Frederick reigned for 60 years before his death at Windsor Castle in 1820.
George was succeeded by his son George IV, and the throne then passed to George IV's brother, William IV, upon his death. An 18-year-old Victoria took the throne upon William IV's death in 1837.
Mann said he was blown away to think such a small piece of history could be hidden in a central Auckland sports field.
"It's one of those things, you think, 'Who dropped it and what was the world like then?'
"What were the circumstances around dropping it?
"It was there before the world had its last pandemic, it was there before Queen Victoria's reign, it was even there before the Treaty of Waitangi was signed."
The British coin was the official legal tender coin of New Zealand until 1933, when the Coinage Act, which governed currency, coinage and legal tender in the country was passed.
It meant that British coin ceased to be legal tender as on February 1, 1935 - making New Zealand the last and most remote of the self-governing dominions of the British Commonwealth to introduce its own coinage.
The back side of the found coin. Photo / Supplied
Mann said it was "extremely rare" to find a coin in such good condition, or of such an age in New Zealand.
"Often with old coins they can be quite smooth, because they're used for so long that they wear down.
"This one must've been dropped quite early in its life because it's still raised."
Likely only to be worth a few hundred US dollars, Mann said it would occupy a special place in his collection.
"I have been detecting for about 3 years, and though I have unearthed hundreds of pre-decimal coins, this is the oldest coin I have ever found," he said.
"The fun of metal detecting is you never know what's under your feet and you never know what you'll find when you're going out."
