Pool queen Emma McKeon's mysterious absence from championships is solved - and it's NOT good news for her Olympics chances

Emma McKeon's Paris Olympics preparations have taken a hit after it was revealed Australia's most decorated Games athlete is battling a torn muscle under her armpit.

McKeon was set to race alongside the returning Cate Campbell in Monday's 100m freestyle heats at the Queensland swimming championships in Brisbane.

But her lane was empty, with the 11-time Olympic medallist's coach Michael Bohl later confirming she had been withdrawn as a "precaution".

Bohl said McKeon had been managing "a minor tear under her armpit for some weeks" and that the medical advice was to not rush back into competition.

The championships at Chandler's Brisbane Aquatic Centre are the last meet of the year.

The Olympic swimming trials will take place at the same venue from June 10, some five weeks before the 2024 Games in Paris.

McKeon won four golds at the Tokyo Olympics in 2021, and seven medals in total.

It was the most by any female swimmer at a single Olympic Games and the joint-most medals across any sport with Soviet gymnast Maris Gorokhovskaya, who achieved the feat in 1952.

Campbell, vying to become the first Australian to swim at five Olympics, has returned to competition this year from an extended break following the Tokyo Games.

She took positive steps on Monday morning, winning her heat in 53.73 seconds, third fastest behind top qualifier Shayna Jack (53.03).

Pop star turned swimmer Cody Simpson, also the boyfriend of McKeon, qualified fastest for Monday night's 100m butterfly final in a boost for his Olympic selection stocks.

He was slightly off the pace in the 100m freestyle heats soon after but still snuck into the final after several withdrawals.

The meet is also serving as a comeback for Ariarne Titmus, who qualified fastest for Monday's 400m freestyle final, an event in which she is the world record holder.

On Sunday night Titmus was relegated to silver in the 200m freestyle, beaten by new world record holder Mollie O'Callaghan.

It's the first racing for Titmus since she revealed in September she had benign tumours removed from an ovary.