Joyce resigned as Deputy Prime Minister and Nationals leader last February after his affair with Ms Campion was revealed.
The couple already has a baby son, Sebastian, and Mr Joyce has four daughters from his marriage with now-estranged wife Natalie.
Opposition Leader Bill Shorten congratulated Mr Joyce and his family.
"That's entirely a personal matter for them and I congratulate them. Whenever a child is to be born it is good news," Mr Shorten told reporters in Queensland on Sunday.
Prime Minister Scott Morrison also offered his congratulations.