The Anglican Diocese of Sydney has donated $1 million to the "no" campaign which is lobbying Australians to oppose legalising same-sex marriage.

Archbishop of Sydney Glenn Davies made the announcement in an address to the 51st Synod of the Diocese of Sydney on Monday, but the donation was given to the Coalition for Marriage about a month ago.

The Anglican Diocese of Sydney is a founding partner of the Coalition for Marriage, the group leading the "no" case in the national postal survey on same-sex marriage.

"The standing committee has also enthusiastically backed our participation in the Coalition for Marriage and has taken the bold step of drawing down $1 million from the Diocesan Endowment to promote the 'no' case," Archbishop Davies said in his address.