Mr McLaren surmises a passenger threw an apple core out their carriage window and one of the pips germinated.
The tree has since died, leaving the McLarens as the caretakers of its progeny.
They have several young trees growing and are propagating more in the hope the variety can be commercialised.
Mr McLaren suspects the tree originated from France and was brought to New Zealand during the goldrush.
Its apples have an elliptical shape and, although not ideal for eating fresh, were excellent for cooking — even better than Granny Smith, he said.
"It makes a beautiful apple pie."
Mr McLaren previously worked for the government in horticulture research, and is a shareholder and director of the Nevis Fruit Co Ltd, which develops apple varieties.
He plans to give the new variety a name — perhaps "Goldminer".