Follow-on an option for Australia in rainy Galle

Steve Smith may need to enforce the follow-on to seal an Australian win over Sri Lanka that appeared guaranteed before rain hit Galle in the Test series opener.

The entire second and third sessions on day three were lost to rain after Australia (6-654 dec) continued their dominance of the hosts (5-136) with two key wickets before lunch.

At around 2:30pm local time, heavy clouds rolled in from the north and forced the covers back on just after they had been removed, with play officially called off around an hour later.

Scattered showers are expected on day four before the weather clears on Sunday, increasing the possibility of a draw if more time is lost.

But Australia are still 518 runs ahead of their hosts after Usman Khawaja (232), Josh Inglis (102) and Smith (141) starred with the bat across a dominant first two days.

Provided their lead remains 200 or more, Australia could take the remaining five wickets and force Sri Lanka to bat again on a pitch already deteriorating.

Australia has historically deployed the follow-on less often than their international rivals, having been on the receiving end of three of the four losses in Test history for teams enforcing it.

In the past 15 years, Australia has only enforced the follow-on five times, with acting skipper Smith in command for one of those matches, a win over the West Indies in Hobart in 2015.

But it may be Australia's clearest path to victory against Sri Lanka.

"It'll be up to Smudge (Smith) and Ron (coach Andrew McDonald)," said Australian spinner Matt Kuhnemann.

"They'll get together tonight and come up with a plan for us tomorrow morning and we'll go from there."

In a bright first session before the rain, Mitch Starc (2-13) and Kuhnemann (2-48) disposed of Sri Lanka's strike weapon Kamindu Mendis and captain Dhananjaya de Silva.

Dinesh Chandimal (63no) offered resistance for the hosts, who had fallen to 3-44 at stumps on day two and were in desperate need of his positive intent.

The 35-year-old wore a bouncer from Starc on his hip and needed medical attention on the field but fought on for a 31st Test half-century.

He looms as the crucial wicket for Australia's bowling attack, having made his highest Test score - an unbeaten 206 - in the sides' last meeting in Galle in 2022.

"He's batting beautifully at the moment," Kunhemann said.

"Played some lovely shots against all of us. It's just whether or not we're trying to block that boundary option for him and get him defending, get him out caught bat pad, slip and whatnot."

Kusal Mendis (10no) is unbeaten next to Chandimal.

But teammates Kamindu Mendis and de Silva threw their wickets away earlier in the first session.

Starc did well to extract some swing from the ball during the session and enticed Mendis into poking down the leg-side, where he was caught by Alex Carey.

Mendis (15) had scored five centuries last calendar year, including an unbeaten 182, the last time Sri Lanka hosted at Galle in September, against New Zealand.

De Silva (22) advanced on Kuhnemann but missed the left-arm offspinner's length ball and found himself stumped by Carey.

Axed from the Test side for the series opener, Sam Konstas watched the third day's play from the team hotel after coming down with a stomach bug. Team officials were confident the issue was not serious.

Australia confirmed legspinner Tanveer Sangha would join the Test squad in Galle as their second development player after Ollie Peake, but is not expected to be considered for selection.