Needling of arteriovenous fistulas is a critical aspect of vascular access in haemodialysis, where a particular concern is the venous return needle strongly perturbing local haemodynamics. To isolate and characterise these effects, controlled experiments are performed in an idealised geometry representing venous return, using clinically relevant vessel and needle dimensions with Reynolds-number similarity. Scalar transport and mixing are quantified using planar laser-induced fluorescence, while velocity field and shear stress are obtained from particle image velocimetry. The results demonstrate that needle angle and the vein-to-needle flow-rate ratio jointly govern the development of primary and secondary flow structures. Lower needle angles and higher flow-rate ratios delay mixing, whereas higher angles and lower ratios promote rapid mixing and sustain localised stagnation regions. Complete mixing, when achieved, occurs within approximately 15 needle diameters downstream, although the velocity field remains undeveloped and influenced by the needle jet up to 25 diameters downstream. Viscous shear stresses are found to be highly unsteady (root-mean-square of fluctuation/mean value varying between 0.21 and 0.48 at the location of maximum shear stress), with instantaneous values frequently exceeding physiologically relevant levels. These findings provide fundamental fluid-mechanical insight for clinically informed optimisation of cannulation strategies.