The Morrison government has forcefully intervened against casual workers "double dipping" on their entitlements in a last-ditch appeal to prevent employers from being held liable for billions of dollars in backpay.

Minister for Industrial Relations Kelly O'Dwyer's submission to the full Federal Court, which is set to determine whether casuals working regular hours can claim both casual loading and permanent entitlements, argues casuals have no case for such a "windfall" and to grant one would go against Parliament's intentions.