Diabetic retinopathy (DR) diagnosis relies on clinical signs visible during dilated fundus examinations. However, pre-clinical DR may have neurodegenerative alterations that are only apparent through visual function assessments. This study compared visual functions in type 2 diabetes mellitus (DM) patients at various non-proliferative diabetic retinopathy (NPDR) stages and determined the risks of reduced visual functions. A total of 56 adult DM participants (mean age: 40.41 + 7.281 years) were classified into DM without DR, mild NPDR, and moderate-to-severe NPDR. Visual acuity (VA), colour vision discrimination (CV)  and contrast sensitivity (CS) were assessed using logMAR chart, FM100 Hue test, and Pelli-Robson chart, respectively. The moderate-to-severe NPDR group exhibited poorest VA than other groups, with reduced CV discrimination (all parameters p<0.05) and reduced CS [F(2,107)=22.898, p<0.001]. An adjusted multinomial logistic regression model revealed a 24.4% higher risk of reduced CS in the moderate-to-severe NPDR group compared to DM without DR group (OR:0.756, 95% CI: 0.627-0.913, p=0.004). In conclusion, moderate-to-severe NPDR had reduced CS and CV, with higher risks of reduced CS compared to those without DR. Incorporating these parameters into current DR optometric screening programs can mitigate NPDR progression and enhance long-term visual function outcomes of the patients.