It was one of the biggest scandals in AFL at the time.
The pair, who had been married for 18 years, had four children together – daughters Lucy and Ruby and sons Oscar and Max.
"It was public, very public," the 53-year-old told Australian entertainer Rhonda Burchmore on I'm a Celebrity... Get Me Out of Here! in January.
Watch: Dale Thomas and Billy Brownless join I'm a Celebrity... Get Me Out of Here! Post continues below.
"She just fell out of love, which happens," he said.
"I didn’t expect it," he continued. "You think she’ll come back and all that, but she didn’t. So that was a real kick in the guts."
Initially, their split didn't garner much media attention.
But around 12 months after their separation, news broke that Brownless' ex-wife had started dating fellow former AFL player Garry Lyon, instantly sparking rumours of an affair.
At the time, the story made front page news in Victoria. Brownless and Lyon both worked on The Footy Show, and had been close friends for several years.
Brownless first addressed the news publicly in a live interview on The Footy Show in 2016.
"I couldn’t believe it," Brownless said.
"I found out three or four months ago that something was going on. I had a hunch. I did some investigation myself, sat Nicky down, and we had a chat and she told me," he continued.