REA opinion - easement devaluation

Interested in the opinion of a real estate agent.
Property is Brisbane within 25km of CBD.

Seller hasn’t disclosed a utilities easement (doesn’t show on title). It cuts through the block in half from the back of the block to the road and is located under the present structure. Concerns are asset maintenance, access for utilities, limitations and restrictions and added cost for future development, perception of buyers on resell.

What discounting factor would you apply and discuss with your seller as reasonable?

I’m the buyer wanting to raise it and discount the price, and want to get into the mind of what the agent might advise their seller.