Whittaker outpointed Romero to win the interim title on a five-round unanimous points decision in Las Vegas last July despite getting injured early in the fight.

Subsequent injuries and illness have delayed the 27-year-old Sydneysider's initial title defence but his bout with Romero will headline UFC 225 in Chicago in June.

Whittaker, who was upgraded to undisputed champion after Georges St-Pierre vacated the full title, was originally scheduled to make his first title defence against American Luke Rockhold in Perth in February.

Health issues forced him out of that bout and replacement Romero scored a spectacular stoppage win over Rockhold.

No.1 contender Romero won an wrestling silver medal at the 2000 Sydney Olympics and at 40 years old remains a formidable foe, as evidenced by his brutal victory over Rockhold and his close fight against Whittaker.

"Romero is a hard fight. It was a war the first time," Whittaker told AAP.

"It's definitely up there as one of my hardest fights to date, one of the hardest fights mentally due to getting injured in the first round and having to get through it all.

"He's such a tough competitor, he's such a good athlete and such a good fighter.

"I'm under no illusion. I think this fight is going to be an an absolute nightmare so I'm going to make sure I'm training my absolute hardest and be the best Robert Whittaker to date."

Whittaker has won his last eight UFC bouts since his last loss in February 2014.

His sole focus is on Romero after he gave up a berth on Australia's Commonwealth Games wrestling team.

Whittaker had a very good reason for not attending the Romero-Rockhold fight.

"Everything happens for a reason, I believe. The week of the fight my little daughter was born," Whittaker said.

"I was home for that but if I was fighting I would have had to have been there and would have missed that."

Australian Associated Press