Yes, but there are different requirements to be met depending on who you are signing on behalf of.
You may sign on behalf of :
Power of Attorney
It is important to note that when signing transfer documents by virtue of Powers of Attorney, there are specific requirements which must be met to be valid for acceptance by the Deeds Office. You should consult a Conveyancer to draft the necessary Resolutions / Powers of Attorney for you, before signing any Offer to Purchase, to ensure it is valid and acceptable.