The court is due to hand down its findings in the landmark case at 12.15pm, however, a judgement on the court's website says the court has ruled against the ACT government’s historic Act, saying it conflicted with federal marriage laws.

Dozens of couples who married in a five day window before Thursday’s ruling have now had their marriages declared invalid.

The Abbott government swooped on the laws the moment they were passed in October, immediately launching action through the High Court to have the Act overturned.

Attorney-General George Brandis urged the ACT government at the time to wait for the outcome of the High Court challenge before allowing couples to marry.

But a defiant ACT government refused and at least 27 couples have married in the ACT since Saturday.

In its case, the Commonwealth argued it had sole power over marriage in Australia and that the ACT laws were inconsistent with both the federal Marriage Act and the federal Family Law Act.

The ACT government argued its law had created a separate status of same sex marriage that could operate concurrently with federal laws.

ACT Chief Minister Katy Gallagher said prior to Thursday’s ruling that, regardless of the outcome, the case had put marriage equality squarely on the national agenda.