Depends on factors not disclosed. Could go either way depending on many things. Lawyer's dream situation - no clear cut answer
BTW are you leasing from a business, or leasing to a business?
Sort of factors considered:
Frequency of previous course of dealings with the business.
Who used to deal with them, if anyone?
What made the employee think he had authority to make the deal?
What made the other business think your employee had authority to deal with them?
What title does the employee have?
How long ago was it done?
Have you now agreed to the transaction with the other business? In other words have you now ratified the contract/lease
Size of contract $?
Is your business a Pty Ltd or partnership or other?