A private agreement is not enough. Only the notarial deed form makes the transfer effective.
The notarial deed is the mandatory form for the transfer of GmbH shares in Austria. A purely party-drafted contract is not sufficient. Send the draft to the appointed notary in good time for formal and substantive review, clarify the identity checks and align the necessary attachments.
Only with the deed executed by the notary does the transfer become effective. Whoever schedules the appointment without a pre-reviewed draft risks rework or an adjournment. A deeper look is provided by the post on buying and selling GmbH shares.