In a notice sent to the ASX, Bill Identity said Optima had 51 customers and more than 196,000 meters under management.
It said the acquisition would be paid with £3.2 million (A$5.8 million) in cash up front, 3384 million shares to be issued to Optima vendors at $1.16 a share (A$3.9 million) and a deferred payment in cash, provided Optima achieved certain performance milestones within a year after the purchase.
Bill Identity will raise $15 million through a share raising, with the shares valued at $1.07 each.
"The acquisition of Optima will accelerate our growth and penetration in the UK market, and offers the potential for expansion in both the UK and European markets.
"We welcome the Optima team and customer base and look forward to bringing value through what will be a best-of-breed robotic process automation platform."
Optima managing director Steven Wright commented: "I am really excited that Optima has joined the Bill Identity family, which is a natural home for the Optima business. Both have a customer-focused approach to delivering innovative software solutions.
"Bill Identity's global client base gives the combined companies the opportunity to deliver a market leading platform for utility bill management, procurement, carbon and sustainability, and tenant on-charging."