What it means
A nullity, or annulment, is a declaration by the Federal Circuit and Family Court of Australia that a marriage is invalid and was never legally effective, under the Family Law Act 1975 (Cth). It is not the same as divorce, which ends a marriage that was valid. Grounds are narrow and include situations such as one party already being married (bigamy), the parties being within a prohibited relationship, lack of genuine consent (for example due to duress, fraud or mistake), or one party not being of marriageable age. Unhappiness, a short marriage, or non-consummation are not grounds for nullity in Australia.
How it's used
Because his spouse was already legally married to someone else, he applied for a decree of nullity rather than a divorce.