The Wiry Wattle is a perennial evergreen shrub that grows to a height of 2 metres (7 ft) tall, although it can grow taller under cultivation. This occasionally weeping bush produces angled glabrous branchlets that are green with yellowish ribs.The foliage are light green filiform pyllodites that are scattered along the branchlets that they resemble, they are typically 6 centimetres (2 in) to 24 centimetres (9 in) in length and 0.75 millimetres (0 in) to 2 millimetres (0 in) in width. A. extensa typically flowers in spring (between August and October) and produces yellow ball shaped blossoms that are generally less than 1 centimetre (0 in) in diameter off short stem stalks called racemes.
A. extensa is found in the South West corner of Western Australia. The species is found as far east as Albany and as far north as Leeman. This species prefers sandy or sandy lateritic soils generally in damp areas such as along water courses or near lakes and swamps.