A mammoth 2.6 kilogram (5.7 pound) lemon the size of a forearm has been grown in a backyard in Western Australia.

Chef Melissa Palinkas excitedly shared an image of the monster citrus to her Instagram last week, snapping the ponderosa lemon next to a regular lemon.

Palinkas, who lives in Western Australia, told the Daily Mail Australia the unusual mega lemon was grown by a local East Fremantle couple in June.

It grows seasonally with a winter yield,' she said.

Intrigued by its mammoth size Ms Palinkas, who prides herself on sustainable produce, said she used the lemon for a salad, after finding it unsuitable for juicing.  

'It is sweet, so I shaved it and made a ponderosa lemon salad with zucchini and green olives with a burnt lemon dressing,' she said.

'The middle part is much like Australia's native finger lime, pearls that burst with juice inside which I sprinkled over the top.'

Palinkas described the pith, which is the flesh of the lemon, as sweet with 'no bitterness at all' and peeled a second ponderosa to brew some fresh limoncello - a popular Italian lemon liqueur.

The ponderosa is a hybrid between a citron and an ordinary lemon.

While not cultivated commercially it is grown by gardening enthusiasts for decorative purposes but can be used as a replacement for lemons in jams, pies and other recipes.