This is a close citrus relative found only in New Caledonia, a french Pacific island territory lying about 1300 km (800 miles) north-east of the Australian city of Brisbane. Oxanthera has a special interest for me because it has some notably similar characteristics to certain Australian and PNG citrus species. The remarkable difference between juvenile and mature foliage is one such feature.
Oxanthera is a genus of flowering plants in the citrus family, Rutaceae. They are endemic to New Caledonia. They are known commonly as false oranges. Phylogenetic analysis has shown these species to cluster in the same clade as Clymenia and the Australian limes within Citrus, which would also make these species members of that genus, and relegate Oxanthera to a subgenus. Species include: Oxanthera aurantium - orange flower oxanthera; Oxanthera brevipes; Oxanthera fragrans - fragrant oxanthera; Oxanthera neocaledonica - large-leaf oxanthera; Oxanthera undulata - wavy-leaf oxanthera.
The following four photos are all from the website of endemia.nc   Please note copyright is CC BY-NC-SA 4.0  Click the photos for bigger and higher definition images.
From the French herbarium of Muséum National d'Histoire Naturelle - click images for original record. This is listed as 'citrus sp.' but appears to be Oxanthera neocalodonica.
The juvenile and mature foliage are from the same plant.
I have no personal knowledge of this species and have never seen any live specimens, so all the information here is gathered from research elsewhere.
©Daniel & Irène Létocart Vue d'ensemble, arbuste en fleurs. 27/07/2004.
©Daniel & Irène Létocart - Rameau en fleurs.
©Jean-Jacques Villegente - Fruit. Tinip. 27/03/2007
©Daniel & Irène Létocart - Détails fleur. Tinip. 27 juillet 2004
Oxanthera neocaledonica

Oueslati, A., Ollitrault, F., Baraket, G. et al. Towards a molecular taxonomic key of the Aurantioideae subfamily using chloroplastic SNP diagnostic markers of the main clades genotyped by competitive allele-specific PCR. BMC Genet 17, 118 (2016).
American Journal of Botany 96(3): 668-685. 2009.
A MOLECULAR PHYLOGENY OF THE ORANGE SUBFAMILY
(RUTACEAE: AURANTIOIDEAE) USING NINE CPDNA SEQUENCES

PHYLOGENY of Oxanthera - Genetic Studies of Relationships
or Citrus neocaladonica
Juvenile Foliage. From 'Ben Caledonica', a prolific photographer of NC plants posted on FLICKR. Click images for originals.

CONTACT ME:
CLICK HERE TO SEND
ME A MESSAGE
Photos & Any further information about Oxanthera is welcomed!
page created 26Nov2021
Click image for large version
These beautiful photos show clearly why Oxanthera plants are known as 'false oranges'. Leaves and flowers could easily be mistaken for C. sinensis. But not the fruit!
From The World Checklist of Vascular Plants (WCVP) 
Oxanthera Montrouz.........................................This genus is a synonym of Citrus L.

Oxanthera aurantium Tanaka..........................................
This species is a synonym of Citrus oxanthera Beauvis.
Oxanthera brevipes B.C.Stone........................................
This is an unplaced name under review
Oxanthera fragrans Montrouz..........................................
This species is a homotypic synonym of Citrus oxanthera Beauvis.
Oxanthera neocaledonica (Guillaumin) Tanaka..........
This species is a homotypic synonym of Citrus neocaledonica Guillaumin
Oxanthera undulata (Guillaumin) Swingle.....................
This species is a homotypic synonym of Citrus undulata Guillaumin
Since Oxanthera has now been placed within Citrus, here are the original and currently accepted botanical names.
However, the descriptions in 'The Citrus Industry, Volume 1' are still probably the most detailed available. They follow the original naming system and are reproduced here from the book.
Click
Text Slideshow Here  or the separate page thumb-nails.