Interim coach Rumesh Ratnayake believes Sri Lanka must have better preparation on fast pitches prior to next year's T20 World Cup in Australia if they are to be competitive in the tournament.

Sri Lanka were beaten out of sight by Australia in the three-match T20I series that concluded on Friday after coming to Australia with high hopes following a series whitewash of the world No.1 team Pakistan last month.

The visitors were badly exposed by Australia's bowlers, scoring just 99, 117 and 142 in the three games, with Kusal Perera the only player to make a half-century. They also struggled to contain the Australian top order who thrived in home conditions with David Warner scoring 217 runs without being dismissed.

Ratnayake suggested Sri Lanka's build-up was inadequate and they need to prepare better for the World Cup next year.

"I thought we gave a good fight," Ratnayake said after the Melbourne loss. "We learned a lot of lessons from this. Our preparation wasn't the greatest. I don't say that as an excuse, but these are lessons that we can learn if we are coming again for a T20 series or any time soon to Australia on a tour. We have to have much more time in preparation in terms of wickets, simulate those sorts of wickets back home and play on those.

"It's just preparation and adaptation. We will be playing a lot in the subcontinent but at least six weeks prior to [the World Cup] if we don't play a small tournament at least in Sri Lanka pertaining to fast wickets, if we don't simulate on fast wickets we'll be in a situation like this. But you saw the curve going well towards the end. But more time spent in Australia, you will sort of learn how and what you should be doing as you go on."

The team's random selection throughout the three-match series raised eyebrows. Oshada Fernando and Bhanuka Rajapaksa had been two of the batting success of the Pakistan tour, and were comfortably Sri Lanka's standouts in the warm-up game against the Prime Minister's XI in Canberra, but both were dropped after the first match in Adelaide despite the bowler's giving up 2 for 233, before being recalled to bat in the lower middle order in the last game in Melbourne.

Lahiru Kumara had bowled impressively from limited opportunities in Pakistan and then his extra pace caused the PM's XI all sorts of bother in Canberra. However, he did not play until the last game in Melbourne where he caused Warner and Aaron Finch more trouble than any other bowler. His figures of 1 for 49 were expensive in the end but he could have had Finch out twice and beat Warner with a cracking delivery in the powerplay.

Legspinner Wanindu Hasaranga, who was man of the series in Pakistan, was also left out in Melbourne just three games later, on the biggest ground in Australia, where Adam Zampa has been extremely effective for Australia.

But Ratnayake insisted that Sri Lanka's selections, led by chief selector Ashantha De Mel, had been designed to give as many players opportunities in Australian conditions as possible.

"His thinking and the thinking behind it has been to give as many as possible exposure because of the coming World Cup next year," Ratnayake said. "We had a choice of either playing Lahiru Kumara in this game or not or playing another seasoned campaigner but we thought if we don't give him another chance in this game then we might miss out on that good exposure.

"He bowled okay, he bowled quick, not the best, but that's a learning curve for him, that's a learning pathway for him. The changes were done so that everybody gets a chance in Australia so that the World Cup will be in Australia in the future."

Sri Lanka haven't qualified for the main draw of the T20 World Cup in Australia next year. They have to play in a pre-tournament round-robin in Geelong against three of the six associate qualifiers, which include Scotland, Oman, Ireland, Papua New Guinea, Netherlands, and Namibia.

Bangladesh and Sri Lanka have been placed in separate pools of four, with the top two teams from each pool progressing to the Super 12s phase.