The influence of pre-metallisation surface preparation on the structural, chemical, and electrical properties of metal-nGaN interfaces has been investigated by X-ray Photoemission Spectroscopy (XPS), current-voltage measurement (I-V) and cross section Transmission Electron Microscopy (TEM). XPS analysis showed that the three GaN substrate treatments investigated, ex-situ HF etch, in-situ anneal in Ultra-High-Vacuum (UHV), and in-situ Ga reflux cleaning in UHV result in surfaces increasingly free of contaminants. Additionally, the three treatments are found to induce increasingly larger upward band bending. Ag-nGaN contacts formed after Ga reflux cleaning exhibit a Schottky barrier height of 0.80 eV and an ideality factor of 1.56, as determined by I-V.XPS and TEM characterisation of Au-nGaN formed after the three pre-metallisation surface treatments show that HF etching and UHV annealing produce abrupt, well-defined interfaces. Conversely, GaN substrate cleaning in a Ga flux results in Au/GaN intermixing. I-V characterisation of Au-nGaN contacts yield a Schottky barrier height of 1.25 eV with very low ideality factor and very good contact uniformity for the pre-metallisation UHV anneal while the Ga reflux cleaning result in a much lower barrier (0.85 eV), with poor ideality and uniformity. I-V and XPS results suggest a high density of acceptor states at the surface, which is further enhanced by UHV annealing. The mechanisms of Ga-nGaN, Ag-nGaN and Au-nGaN Schottky barrier formation are discussed in the context of the Metal-Induced Gap States model (MIGS) Unified Defect Model (UDM) and Cowley-Sze model.