What are the rules on concussion replacements?

If the relevant medical personnel formally notify the match referee that a player has been diagnosed with concussion, the team may activate a Concussion Substitute for the remainder of the match.

• The diagnosis must be notified orally to the match referee (or to the umpires, to relay), then followed up in writing, with a copy of the completed Head Impact Trauma Report Form.
• The team's captain (or coach if the captain cannot) notifies the match referee and nominates a like-player to the opposing captain for approval, which must not be unreasonably withheld. If the opposing captain does not approve, the match referee makes the final determination in consultation with Cricket Australia.
• Once approved, the substitute is a complete replacement and can captain, bat, bowl, keep wicket or field as though a member of the starting XI.
• The team is not obliged to activate a substitute, and there is no maximum number of Concussion Substitutes in a match.
• The concussed player may take no further part in the match.
• On-field assessment is allowed a maximum of 5 minutes.

Source: Cricket Australia, 2024-2025 Sheffield Shield and Second XI Playing Conditions, Appendix C.