Hi Neil,
You cannot bulk update email addresses in the Contact app. A solution is to use a shortcut (in the Shortcuts app) to process the data.
Here is a shortcut I created for this task: Bulk Edit Contact Email Address.
After downloading the shortcut, you will need to edit it to specify the email server to replace, i.e "abcde.com" in the the 1st Text action in shortcut with "company.net.au" (using your example), and the new email server, i.e "fghij.net" in the 2nd Text action of the shortcut with "business.net.au" (using your example.) Please note that the 1st server address is case-sensitive in the shortcut, meaning that replacing "ABCDE.com" (instead of "abcde.com") would not find the contacts to update. (the 2nd server address does not have this limitation.)

I successfully tested the shortcut by adding 2 new test contacts to my contacts. They were correctly updated. However, I would suggest backing up your Mac before running the shortcut. One can never be too prudent.