"Thank you to our incredible OB Ellie Bailey and the wonderful midwives at the SAN for getting our little man earth side safe and sound," the post continued.
"We are so excited to begin this new chapter together as a family of four."
Friends and fans were delighted by the news, flooding the post with messages of love and support.
Reporter Belinda Russel commented: "And then there were FOUR!! Such beautiful photos. Congratulations!!!! xxx"
Spencer is the couple's second son together, after Fiona gave birth to baby Hunter in March 2021.
“For 2 years you were a dream, Over 9 months we watched you grow. In a second you stole our hearts, And showed us a love we did not know,” she wrote on Instagram at the time.
"Introducing Hunter William Falkiner. Born 22.3.2021. 3.3kgs of baby gold. Mummy Fee & bub are doing fantastic.
"A huge thank you to our incredible OB Ellie all the wonderful midwives of SAN Maternity who helped us navigate these first few days of motherhood."
Despite their dreams of motherhood, Fiona and Hayley struggled to begin a family due to fertility issues but Fiona finally fell pregnant through IVF in 2020.
After such a long road, they were delighted when Hayley was able to fall pregnant the following year.
Now their little family is rounded out to four - congratulations again to the proud new mums.