World and Commonwealth Games champion Mollie O’Callaghan has touched out training partner Shayna Jack to win a gripping women’s 100m freestyle final at the Australian swimming championships.

After turning fourth at the 50-metre mark, O’Callaghan mowed down Jack who led for everywhere except the final stroke at the Gold Coast Aquatic Centre.

O’Callaghan produced the world’s fastest time of the year in clocking 52.63 seconds – 0.1sec ahead of Jack.

Confirming the remarkable depth in Australia’s women’s 100m freestyle ranks, Olympic champion Emma McKeon finished third in 53.22.

Former world record holder, world champion and Tokyo Olympics bronze medallist Cate Campbell was equal fifth in 53.78 as her pursuit of a fifth Olympics appearance in Paris takes shape.

The nationals mark Campbell’s second outing since Tokyo two years ago, following her return in a low-key Brisbane meet last month.

Campbell pondered her future after Tokyo but returned to the elite program at a national training camp on the Gold Coast in February.

She is aiming to become the first Australian swimmer to compete at five Olympics.

In other women’s finals, triple Olympic backstroke gold medallist Kaylee McKeown opened her campaign in fine style in winning the 200m individual medley while Ariarne Titmus took out the 800m freestyle final.

Jenny Strauch claimed the 50m breaststroke ahead of Mia O’Leary.

Sam Short swum the fastest time in the world this year for a stirring victory in the men’s 400m freestyle.

Short, 19, clocked a personal best by more than two seconds to win the title in a time of 3:42.46 – the fourth fastest by an Australian – and quicker than Australian all-time great Grant Hackett.

Only Olympic champions Ian Thorpe (3:40.08) and Mack Horton (3:41.55) and reigning world champion Elijah Winnington (3:41.22), who Short beat in the final, have swum faster.

“I really pushed that second and third 100 metres and put everything I had left into the last two laps and it ended up being a two-second PB,” Short, the Commonwealth Games 1500m champion, said.

“I was not expecting that.”

The men’s 100m breaststroke was a triumph for Olympic 200m champion Zac Stubblety-Cook who powered home over the final 25m to defend his title in 1:00.07 ahead of rising star Joshua Yong.

Ben Armbruster turned disappointment into triumph in the men’s 50m butterfly.

Disqualified in the heats but re-instated into the final, Armbruster won in an Australian all-comers record of 23.05.

Bradley Woodward held off a late challenge from Josh Edwards-Smith to win the men’s 200m backstroke.