Quotes from The Citrus Industry, Volume 1:
"The fruits are small, globose, with six segments." 
"The small globular fruits, the elongate-elliptical, crenulate leaves, and the narrowly subcrenulate-margined, very short petioles separate this New Guinea species very clearly from the other species of Microcitrus.   This is the only species of the genus outside of Australia, and it occurs nearly a thousand miles away from M. garrawayi, the Australian species of Microcitrus growing nearest to New Guinea."

Note that the above was written before the discovery of Microcitrus papuana (Citrus wintersii) & Citrus wakonai in Papua New Guinea, and Citrus gracilis in Queensland.
Marc Harris, a visitor to the Riverside collection, kindly sent me these pictures of the fruit he collected there.   Scale is marked in millimetres.
These photos were sent from the University of California 's Riverside Collection.  At times, their specimen is said to have some dieback, especially on the north, and  possibly due to wind.
Common Name: New Guinea wild lime
This picture of fruit and leaves, from the
University of California, Riverside, is also reproduced in The Citrus Industry Vol 1 - but not in colour.
The following images are reproduced from a specimen in Canberra Herbarium, collected in 1984.

As you can see these leaves and fruit match the Riverside pictures very well.
The herbarium specimen text reads:
Undisturbed primary forest, Milne Bay Province.
Medium tree, straight boled 15m tall. Smooth bark, leaves glossy green above, pale green below. Fruit green probably ripening yellow.
All these photos were taken in Papua New Guinea in 2014. See 'The Quest for Wakonai'. - page 19 for further details.
The tree is some 12m. tall. Seedlings nearby have a completely different leaf-shape, a feature common to several species of Australian & PNG citrus.
You can click on these photos for higher definition versions!

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