Sri Lanka should have been closer to the 300-runs mark after making a strong start to Monday's ICC Cricket World Cup match but they lost their way in the middle overs when Australia's bowlers found their range, batter Pathum Nissanka said.

Openers Nissanka (61) and Kusal Perera (78) combined well in a 125-run opening stand that put Sri Lanka in the driving seat in Lucknow, but a batting order collapse then saw them lose all 10 wickets for just 84 runs to post a below-par 209 total.

Half-centuries by Mitchell Marsh and Josh Inglis helped Australia chase down the target with nearly 15 overs to spare, handing Sri Lanka a five-wicket defeat, their third straight loss at the tournament.

"Despite our promising start, we deeply regret not being able to sustain it, resulting in us being limited to a score of 209 runs," Nissanka told reporters on Monday.

"On a wicket like this, I believe we should aim for a total closer to 300 runs, and this was a contributing factor to our defeat.

"The opposition's bowlers consistently hit good areas during the middle overs, which created a challenging situation for our batsmen. Unfortunately, we couldn't handle it as effectively as we would have liked."

Nissanka's knock of 61 from 67 balls was his second successive half-century and the 25-year-old said he hoped to keep piling up the runs.

"I'm hopeful that I can contribute significantly by scoring a substantial number of runs and playing substantial innings in the upcoming matches," Nissanka said.

"Our mental level is good. We hope to play well in these matches and win the remaining matches."

Sri Lanka next face the Netherlands in Lucknow on Saturday.