It may be that your employment agreement with them already states that you can terminate their employment only with [insert period] notice and no further justification required.
If you don’t have such a clause, you will need to take your employees through a change process regardless of their employment status (casual, fixed term, permanent)
An example is below, though it doesn’t have to be spread over a long period of time, this could be done over 3-5 days as long as you have given the employee a written heads up you will be meeting to discuss this as a possibility, allow them to have a support person present (which may be tricky in the current circumstances) then allow them time to respond after you have proposed the change.
Step 1 – Document your business case (reasoning why this is occuring)
Step 2 – Document your proposal (Can be a brief powerpoint presentation)
Step 3 – Present your proposal to employees
Step 4 – Gather feedback
Step 5 – Genuinely consider the feedback
Step 6 – Confirm the new structure – in writing and via a meeting or technology
Step 7 – Implement the change – and keep talking with your staff
Let me know if you need any further information or any further level of detail. Happy to help!