You can surprise your guests, but both people marrying must know about and freely agree to the marriage. A wedding cannot be sprung on an unsuspecting partner in place of proper consent and preparation.

You still need the NOIM, the required notice period, document checks and two suitable witnesses. Your celebrant must meet separately and in person with each of you before the marriage.

If you are planning a party that becomes a wedding, tell your celebrant from the start. Agree on how the marriage will be announced, who the witnesses are and how the legal ceremony will be clearly heard and understood. You can keep the guest list small or celebrate privately.