Australia's freestyle star Lani Pallister has collected gold and silver medals in her arduous opening to the Pan Pacific championships in the United States.

Pallister triumphed in the women's 200m freestyle and then set a personal best when finishing behind American legend Katie Ledecky in the 1500m freestyle in Irvine, California on Wednesday (Thursday AEST).

Dolphins teammates Sam Short (men's 800m freestyle), Brittany Castelluzzo (women's 200m butterfly) and Ben Armbruster (men's 50m butterfly) also triumphed on the opening night of the Pan Pacs.

The Dolphins won four of 11 gold medals on offer, with Pallister's remarkable year continuing.

Just weeks after winning four Commonwealth Games gold medals in Glasgow, Pallister pipped Canadian ace Summer McIntosh in the 200m final.

The Australian clocked one minute 53.67 seconds - just 0.02s outside her personal best in the event - with McIntosh second in 1:53.86.

"I said to Dean (Boxall, coach) at the start of the week that racing this week at Pan Pacs is kind of the first taste of the Olympics in two years time," Pallister said.

"I often don't swim the 200 individually, so going 1:53 is wicked here."

About two hours later, Pallister then returned to the pool for the gruelling 1500m freestyle and set a her best time, and an Australian record, over the distance.

In what was essentially a match-race against peerless American great Ledecky, who hasn't lost in the event in 16 years, Pallister put in a gallant effort.

The 26-year-old shadowed Ledecky, the winner of nine Olympic and 23 world championship gold medals, for the initial 500 metres.

The American great then produced her renowned burst to create a handy advantage en route to touching in 15:36.09.

Pallister's 15:38.62 eclipsed her previous best of 15:39.14 from June last year.

In the men's 800m freestyle, Short went close to a personal best as he edged American rival Bobby Finke.

Short clocked 7:37.02, just 0.71s shy of his best time in the event, with Finke finishing some five-and-a-half seconds behind in second.

Earlier, Armbruster claimed Australia's first gold medal of the pool meet, winning the men's 50m butterfly in 22.75 seconds - 0.02s outside the Australian record held jointly by Kyle Chalmers and Matt Targett.

And Castelluzzo took out the women's 200m 'fly in 2:06.24 from her Dolphins teammate Liz Dekkers (2:06.56).