The long-time workers' advocate yesterday announced his resignation, after 13 years as assistant secretary, to take up a role with a not-for-profit organisation.

The 53-year-old said the decision had been difficult, particularly after a lifetime of union work.

"It's really hard to walk away; I came up from the shop floor and worked my way through the ranks to become the assistant secretary for Unions NSW in the mid '90s," he said.

"To have been in this position and then to leave, after 13 years - it's incredibly hard but I'm looking forward to a new challenge."

Motivated by his mother's battle against workplace discrimination, Mr Christodoulou first joined a union at the age of 16 while working as a Coles trolley boy.

"I wanted to change the world," he said.

"I really wanted to make it a better place for workers; I'd seen how little power they had against big companies and that has always driven me over the years.

"I saw that workers, particularly migrants and low-income earners, really needed a strong voice. Being in the union and having wins for them, I'm really proud of that."

Selecting a single career highlight from decades of industrial relations stoushes and successes was difficult for Mr Christodoulou.

But he named a historic win for the rights of people with disabilities and his organisation of industrial arrangements at the 2000 Sydney Olympic Games as stand-out moments.

Mr Christodoulou was also chairman of the Illawarra Venues Authority (which has been replaced by Venues NSW) for more than a decade.

In his time on the board, he oversaw the construction of WIN Entertainment Centre, WIN Stadium's northern grandstand and the western grandstand.

He also fronted the media after strong winds buckled the western grandstand's roof.

"It was disappointing but as chairperson I had to say something," he said.

"The government wanted to hide away from the problem but I didn't want that, so I went out and said 'there is a problem'; unfortunately my time on the board was up before I ever found out what went wrong."

Although he could not disclose details of his new venture yesterday, he said he would continue to advocate for workers' rights.

"I've seen the massive benefits that unions have fought for and I think it's something workers today really take for granted."

"I will never give up the fight; it's something I've stuck by all through my career; you have wins and you have losses but the losses should never make you less determined to fight."