"...but do i also need to reduce my cost base for each remaining...

"...but do i also need to reduce my cost base for each remaining share i hold ?"

I expect that the ATO will put out a "class ruling" on what the ATO sees as the correct taxation treatment of the return of capital by EZL..
I haven't been following the EZL matter that closely, but I expect that the short answer to your question is yes, the cost base of EZL shares will be reduced by the return of capital.

I am not sure if the ATO produce such class rulings off their own bat or whether the companies that are undergoing some form of restructering ask the ATO for a class ruling for the benefit of their shareholders.
I suspect it may be the latter.

As an example, there are ATO guides on the tax treatment of the various restructerings affecting Wesfarmers.

Cheers