Harding died aged after being diagnosed with breast cancer which then spread to other parts of her body.
Posting on Instagram on 5 September, Walsh paid an emotional tribute to Harding, writing: "It feels more real than ever today."
She also said: "Sarah passed away a year ago.
"I can still hardly believe it to be honest but it feels more real than ever today. Thinking of her always and of everyone who loved and miss her too."
Peter Andre also wrote a tribute to Harding as part of his column in OK Magazine. Andre wrote: "It was such a shock when Sarah died, but what I love about the Great British public is that when a terrible situation like this happens and someone so loved passes, people put all their love and support behind the family."
The popstar added: "Sarah was a very loved person, she will never be forgotten. I hope her family have been able to in some way come to terms with their loss. It’s such a terrible thing to have happened to someone so young."
It was the group's first public appearance together since 2013.
At the time, Cheryl talked about dealing with the grief of Harding's death: "You know, I've lost grandparents when I was younger and I lost a friend when I was younger but nothing like this, the feeling of shock and disbelief and it still lingers now to be honest. I still can't quite believe it's real."
The remaining members of Girls Aloud are planning other cancer fundraisier activities including a gala event later in 2022.
Watch below: Girls Aloud reunite for the first time in nine years in memory of Sarah Harding.