Formerly Microcitrus australis, this is commonly known as the Australian Round Lime or sometimes by one of its Aboriginal names - Dooja.
It occurs as a shrub or tall narrow tree, with glossy, dark-green leaves, on the more open and drier rainforest margins of southeast Queensland, from the Brisbane area and northwards. The round fruits, about 2cms to 5cms in diameter have a thick, green to lemon-coloured skin and pale green pulp. It looks very similar to a small, bumpy lime.
I took these photos, with unripe fruit, at  Villa Hanbury
in Italy, where there is a very fine specimen - a rare European example of this Australian species.
There is also a good fruiting specimen in the Propagation Dept. of the New York Botanical Garden, N.Y., USA. This area is closed to the public, but if you ask nicely, a member of the staff would probably show you the plant.
The New York tree is about 11' tall in a 2' diameter pot. It appears to be a seedling (no graft visible) acquired in 1979. The July fruits are not yet ripe. Notice the diamond shaped leaves, less rounded than on the older Hanbury tree.
On a visit to Australia in October 2016, I located this wild specimen growing about 40kms south of Brisbane.
For details see ....finding Citrus australis
This label for 'Australian Round Lime' is from Daley's Fruit Tree Nursery in Kyogle, New South Wales.
The photo is actually one of my photos of the tree at Villa Hanbury in Italy. At the time the nursery contacted me in 2004, they could find no Australian photos of this species!
The Round Lime develops several distinctly different shaped leaves as the plant matures. (Funny that this seems to be a particularly Australian plant habit - many Eucalyptus have differently-shaped mature leaves.) The first three photos were taken in Australia of a wild Round Lime, the fourth is of the old tree at Villa Hanbury
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Take a look at my new page ....finding Citrus australis