As there is a range of level of hearing impairment exactly how you would manage the risks associated with the work will depend on the worker. Making sure there are clear visual warnings (flashing lights, high vis etc) will be more important as will be establishing an agreed form of communication between workers to enable them to communicate instructions effectively.

You could try contacting Workbridge - https://workbridge.co.nz/employer-resources to see if they have any resources or information they can make available to you.

Having played sport with hearing impaired people I found they were easy to integrate and once people understood what was needed it was easy to manage. The last part of the sentence is key as you will need to train and assist your other workers to help them understand what is required to help the new employee.
Finally, as an aside as discrimination for disability is a prohibited ground of discrimination under the Human Rights Act that may be where Keith is coming from.