Phlebopus aff.marginatus is a massive, robust bolete, often producing some of the largest mushroom fruitbodies in Australasia. Caps are broad, thick, and fleshy, typically brown to yellow‑brown, smooth to finely tomentose, and may reach 300–500 mm across in exceptional specimens. The margin is thick, rounded, and often slightly inrolled when young.

Cap: Large 150-230 mm, convex to plane; brown, yellow-brown, or olive-brown; surface dry, smooth to finely velvety; margin thick and blunt.
Cap going blue at the top of the stipe when cut in half.
Pores: Yellow to olive-yellow, large and angular; do not blue when bruised; tubes deep and firmly attached.
Stipe: 100-150 mm, Thick, stout, often swollen, sometimes short relative to cap size; coloured pale yellow-brown to brown; surface smooth or slightly fibrillose.
Flesh: Pale yellowish, firm, very thick; no bluing.