Hi Mark74
Under copyright law, the designer themselves will own copyright in the pattern by virtue of them being the creator of the pattern. The only way that ownership of the copyright could be held by you is if the designer assigns the copyright to you by way of a deed of assignment.
However, copyright law will only protect the actual pattern itself from being reproduced (ie the designer making a photocopy of the pattern). It will not protect you from a garment being made from that pattern, or from a copy of that garment then being made.
Another way of approaching your concerns might be to put an exclusivity arrangement in place between you and the designer, in which the designer agrees not to use the pattern for any other customers. I would recommend talking to the designer direct about what agreement can be reached to protect your interests.
You can read more about copyright law here:
Copyright | IP Australia
I hope this has helped,