Gossia (Myrtaceae) is a highly restricted tree genus most speciose in New Caledonia and eastern Australia. The latter group accumulates above-normal foliar manganese (Mn) concentrations, with some individuals exhibiting the rare Mn-hyperaccumulative trait. Whether foliar metals contribute to chemical defence has been addressed via numerous feeding experiments and very few field studies. This investigation exploited specifically different insect activities on the foliage of sympatric Gossia grayi (N.Snow & Guymer) and Gossia shepherdii (F.Muell.) N.Snow & Guymer, endemic to north-eastern Australia, to test for direct and indirect effects of foliar Mn enrichment on plant–insect interactions. Leaf organic and inorganic chemistries, specific weight, surface damage, gall infestation and occupancy were quantified. Discovery that both species are Mn hyperaccumulators augments the world listing by 5–7%. Highly elevated gall-Mn concentrations coupled with negligible gall parasitisation suggested chemical fortification and adaptation by the host insect – a Cecidomyiidae fly. Linear mixed modelling (LMM) showed differences in leaf Mn, phenolics, toughness and surface damage across tree species and leaf age. There was no direct relationship between leaf Mn and insect impact. However, LMM did resolve indirect effects, i.e. between insect impact and certain foliar elements, consistent with nutritional dynamics in a physiologically novel plant system where Mn is vastly overaccumulated.