Cupronickel alloys, reinforced by L1$2$ precipitates, offer a high strength, corrosion resistant and anti-bio fouling material for marine engineering applications. These alloys exhibit complex precipitate nucleation and growth mechanisms that must be fully understood in order to optimise the mechanical properties. In this work, the microstructural characteristics of the Cu-Ni-Al based alloy, Hiduron$\mathrm{<mrow\; data-mjx-texclass="ORD">\circledR </mrow>}$ 130, have been assessed in the as-received condition and following 100-h thermal exposures at 500, 600, 700 and 900 °C and after a two-stage exposure at 900 °C for 100 h followed by 500 °C for 100 h. The L1$2$ phase was observed to precipitate both discontinuously and continuously and its subsequent coarsening was characterised alongside measurements of the lattice misfit. The hardness of the alloy was found to decrease with increasing exposure temperature up to the L1$2$ solvus and correlated with the evolution of the L1$2$ precipitates, the alloy grain size and the fraction of the discontinuously formed L1$2$ phase.