Department store Myer and womenswear designer Kym Ellery have commenced a three day showdown.
A three day trial between the warring traders kicked off in Melbourne today.
The case is expected to set a landmark precedent regarding exclusivity terms between suppliers and retailers.
Myer is seeking an injunction preventing Ellery from supplying her mainline and diffusion label L'America to rival store David Jones.
Myer is also seeking damages against Ellery, claiming the designer breached an exclusivity contract and failed to deliver stock for autumn/winter 2013.
It us understood Ellery signed a three year exclusivity contract with Myer in 2011, before defecting to David Jones with a $500,000 contract late last year.
Court documents reveal David Jones sent Ellery an exclusivity letter on November 7, after an initial meeting on August 16.
It is understood the letter was signed two days later without Myer's knowledge.
The trial comes less than a week after Ellery unveiled her spring/summer 2013/14 collection at Mercedes-Benz Fashion Week Australia.