About 150 residents and visitors to the Lawrence Heritage Day joined the celebrations as a gap in the weather cast sunshine on the former swimming pool frame, bringing to a conclusion a $75,000 project conceived in 2016.
Mrs Hay said it was "excellent'' to see her idea come to fruition.
"I wasn't expecting such a celebration of people. The Arches look magnificent against the blue sky and people seem very pleased with the result.''
Mr Cotton paid tribute to Mrs Hay's "drive'' in realising the project, and to the many donors and volunteers who had helped achieve her vision.
"Although the planning took a while, it's come to life pretty smartly. It's a big transformation of a disused building into a community asset, and we hope people continue to use and enjoy it into the future.''
Mr Cotton said he envisaged the facility as a space for relaxation, and for use for functions such as weddings.
The site would be illuminated at night.
