What are the fees like for public schools in Perth?

Yes. School fees for Primary schools are usually between $50 to $100 per child as well as a P&C levy (Parents and Citizens fund raising organisation ) of between $20 - $100 depending on the school. My fees are $60 school fees and $50 P &C levy. Some schools offer a discount for more than one child.

High schools are quite different and more expensive. My fees for a year 9 child are $550 including course fees, school board and other fees.

The basic details for each government school can be found on the website below.
http://www.det.wa.edu.au/schoolsonline/home.do

It's not voluntary. There are just no consequences if you don't pay it - at least not to yourself as an individual. That money however is included in the budget for the school. If enough parents don't pay it, teachers have to cut services. This means no new books, or no excursions. The government does not make up the shortfall. Obviously this can have a substantial impact in some low socio-economic areas where very few parents pay. Please pay it - as others pointed out, it's not an unreasonable amount!

There is a voluntary fee. I think it's $65 or $100 for two kids. I pay it and I think most would as it's very reasonable and most people have this money to spare.