Proteaceae are adapted to acquire P from nutrient‐impoverished soils; many function at very low leaf P levels, but are killed by P fertilization. Phosphorus toxicity develops at a remarkably low external P concentration. Previous studies have described P toxicity in Proteaceae, but the physiological basis for it remained unclear. The aim of the present study was to elucidate the physiological basis of P toxicity in Hakea prostrata R. Br. (Proteaceae). Triticum aestivum L. (Gramineae), Medicago truncatula Gaertn., Lupinus albus L. (both Fabaceae) and Hakea prostrata R.Br. were grown in solution at a range of P concentrations (0–1000 mmol P m⁻³), and determined net P‐uptake rates at 5 (all species) and 50 mmol P m⁻³ (H. prostrata only). With the exception of H. prostrata, net P‐uptake rates were fastest for plants grown without added P. Down‐regulation occurred for T. aestivum, M. truncatula and L. albus when the P concentration during growth was increased from 0 to 0.8 mmol P m⁻³, whereas in H. prostrata rates decreased only for plants grown at 10 mmol P m⁻³ or more. The leaf [P] at which P toxicity occurred in H. prostrata exceeded 10 mg g⁻¹ dry matter, similar to that for crop species. The low capacity to reduce P uptake in response to increased supply offers a physiological explanation for the extreme sensitivity to P supply in H. prostrata, and possibly other Proteaceae.