A murder probe is underway after a woman was found dead at an apartment block in Perth's north.

Police and forensic crews have been at the unit on Danehill Way in Balga since 9am today.

A man is in custody and police say they are not looking for any other suspects.

Forensic police at the Balga unit block today. (9News)

A man is in police custody. (9News)

The cause of the woman's death is not yet known.

9News understands paramedics were called but the woman could not be saved.

The apartment block will remain a crime scene overnight.