A landmark ruling will see long-term casuals given access to paid annual leave.
Typically, casuals get paid an extra 25 per cent to compensate for a lack of workplace benefits, but unions argue many are exploited, because they work full time hours with no entitlements.
The federal court ruling will give casuals paid annual and sick leave, if they consistently work full-time.
Employers could be exposed to back pay of up to $8 billion.
Business groups warn it could send many operators broke.
Unions are welcoming the decision, while the federal attorney-general says the government will consider employer pleas for changes to the Fair Work Act.