Casual are great for flexibility. you pay for the hours you...

Casual are great for flexibility. you pay for the hours you need. You need someone to mow a lawn, you pay for the lawn to be mowed. if you have a project say 3 months, you employ a casual for 3 months. Its the same as contractor.
They know its casual work and a timeframe, the casual get paid casual loading, say 25% which effectively covers perm entitlements.

Unfair dismissal still applies to any employer, casual, perm, contract... anyone.

With the legislation:
It will be the same for recruitment companies i guess, if a employee of the recruitment company is employed in ongoing work (or a certain amount of hours) for 12 months they will be obliged to offer them permanent position under the recruitment company name. they will still be a contract to client however recruitment company has ownership in a way.