I love the Beacon Yoga Centre ( http://sivanandaashram.org.au/ ), at Beaconsfield, just outside of Fremantle. It's situated in a lovely rambling old building with views over the ocean, and has loads of atmosphere. There are plenty of classes (including ones for total beginners), five days a week, and unlike some yoga studios they provide yoga mats and blankets.
Little Mumma Yoga is an awesome yoga class for mums with bubs who are not yet crawling. Great for relaxation and post-natal exercise in a supportive environment. Held in 2 locations south of the river in Palmyra and Success.