Not every job needs the top of the range light fitting so here are some thoughts on what you are paying for. One of the worst enemies of light fittings is heat. This is why it is a bad idea to put in a stronger bulb than the manufacturer recommends. Even with the right bulb heat will cause low quality lamp holders to fail, and worse the insulation on the power supply wires to break down.
A minimum should be a porcelain lamp holder with the wires connecting it covered in heat insulation.
If the fitting is made of plastic the heat will make it brittle over time and if it is made of zinc alloy the heat may cause it to distort.
These problems are minimised by using polycarbonate or aluminum castings, which of course means the fitting will cost more but anyone who has had a bulb stuck in a seized socket or has a down light not sitting flush will think it worth it.
All metal fittings should be earthed or double insulated and will be if they meet the AS/NZ standards which will be the case for electrical fittings sold in NZ. It is external lighting where low cost products can let you down. Outside you still have the heat from the lamp and in addition, rain,sunlight and possibly salt air.
Ultra violet light from the sun breaks down plastic and low quality powder coating. Zinc alloy corrodes, and this can be rapid if it is in contact with acid soil.
Aluminum with good quality powder coating will stand up well but if it is in contact with a dissimilar metal the electrolytic action will have it crumbling away in no time. Good idea to use stainless steel screws to fix the light and definitely not mild steel screws.
The best materials for external lighting are the brass, copper and stainless steel lights, but again the best quality is expensive. If the fittings are lightly built to save material they will not absorb the heat as well as a robust solid light.