From 1 March this year, the firm will merge with Flower & Hart in Brisbane. Flower & Hart will rebrand as Middletons.
"Growth in strategic markets has been a key theme at Middletons for a number of years. This piece in our domestic footprint represents our commitment to being available to support our clients in all major markets," said Middletons managing partner Nick Nichola.
It is understood the firm was already finding work in the northern state, but expects the merger will add a more established commitment there.
The firm is expected to quickly boost the Brisbane office's energy and resources practice, as well as its workplace relations and safety practice.
In less than five years Middletons has doubled its geographic footprint, despite tough market conditions and declining business confidence.
Middletons' merger with Salter Power in Western Australia in 2008 has been a success story, the firm said, which highlights the firm's strength in managing its growth.
Middletons said it's confident that in Flower and Hart, the key attributes for a successful merger of compatible cultures, mutual commitment to quality, outstanding reputation built on highly responsive client service and quality partners and staff, have been satisfied.