Nosings on Stairways
A highlighted nosing marks the leading edge of each stair tread. Without it, someone with a vision impairment can struggle to distinguish where one tread ends and the next begins, in poor light a stairway without nosing highlights can read as a ramp or a shaded section of walkway.
The BCA covers stairways at D3.3(c) and calls up AS 1428.1 wherever a stairway is required to have access features.
Getting it right
The contrast strip needs to run continuously across the full width of every tread, at a depth of 50–75mm, with adequate luminance contrast to the tread colour. The failures worth watching for: nosing treatments that lose their contrast when wet, and thin slip-resistant inserts that don't actually reach the required 30% luminance contrast.