Who is responsible for a possibly concussed player when there is no doctor at the match?

The club or team. The Hockey Australia Concussion Policy (effective 1 January 2024) says that at matches with no dedicated match-day medical professional at the venue:

• The welfare of each player is the responsibility of their club or team through its nominated representative, who may be a coach, manager, captain or official.
• If a player shows any visible sign or symptom of concussion after a head impact or collision, play must be stopped.
• It is not the match officials' responsibility to assess players for injury. They can stop the match for an injury, including a suspected concussion, subject to the rules of hockey for that match, but the club or team remains responsible for the player's safety and welfare.
• The nominated representative (or, if there is none, the coach or captain) must make sure the player is removed from the field before play recommences.

If you are your team's nominated representative, these duties fall to you.