Is it possible to employ someone on a fixed-term contract on a 6 month rolling basis (i.e continue renewing the fixed-term contract every 6 months)?
Yes, you can employ someone on that basis. However, you should tread carefully if you decide to do so.
Once you renew or extend a fixed term contract one or more times, you create in the employee an expectation that it will be renewed on the next occasion. If, contrary to that expectation, you decide to let the contract expire, the termination of employment may be found to have been initiated by you – which means the employee might be able to access redundancy or notice entitlements or even make an unfair dismissal claim.
If the main reason you are engaging employees on a fixed term basis is to avoid unfair dismissal laws, or notice and redundancy entitlements, and there is no operational reason for this type of engagement, then you will have to treat the employees as if they were engaged on an ongoing basis.