Thomas O’Hare, 33, and his girlfriend Lisa McClatchey, 21, died when they were attacked at their home at Tassagh, near Newtownhamilton, in November 2006.
Petrol was poured around the house and set on fire.
The arrested man, who is from Armagh, was detained by Australian police following a request by officers from the Police Service of Northern Ireland’s Serious Crime Branch.
He is due to appear in court in Sydney today to face extradition proceedings.
Earlier this month, two brothers were charged with double murder and arson in connection with the killings.
More to follow.