Perth freeway bus crash cause unknown

It is still not known what caused a bus to cross onto the wrong side of Perth's Kwinana Freeway, killing a mother and injuring her young daughter.

The TransWA bus was heading south about 1.10pm on Tuesday when it crossed the median strip near Paganoni Road in the southern suburb of Karnup, into an oncoming 4WD.

The 42-year-old driver was killed and her four-year-old daughter taken to Princess Margaret Hospital with non-life threatening injuries.

The 63-year-old male bus driver and a passenger in her 20s were also taken to hospital.

The remaining 21 passengers were treated at the scene for minor injuries.

Police do not know what caused the crash and their investigation is continuing.