Flax lily (Dianella ensifolia)

The plant you're referring to is likely the Flax Lily (Dianella ensifolia). This perennial thrives in swampy regions and can reach heights of up to 1.5 meters. It spreads through its roots to create extensive clumps. The juice extracted from its berries has traditionally been used for dyeing textiles and garments, and it is potent enough to stain surfaces like pavement.