'The size of a standard school ruler': The most terrifying component of the Thai rescue dive.

With AAP. 

The cave rescue of 12 Thai boys and soccer coach could take two-to-four days, a Thai official says.

The dangerous rescue mission to free the group trapped inside a Thai cave for two weeks began at 10am on Sunday (1300 AEST) with hopes the first survivor to possibly emerge 11 hours later (midnight AEST).

Eight Australian divers are involved in the treacherous mission and a team of Australian medics assessed the boys’ condition before the rescue effort got the green light on Sunday morning.

A Thai army commander says the rescue could take two-to-four days, depending on conditions inside the partially flooded cave.

According to Major General Chalongchai Chaiyakam, the 13 “will continuously come out in approximately 2-4 days, which all may change depending on weather and water conditions”.

Earlier head of the rescue mission, Narongsak Osottanakorn told reporters:”Today is D-Day”.

“The weather is good. The water level is good. The divers are ready. The boys are physically, emotionally and mentally ready to come out,” he added.

The operation will see each boy escorted by two divers, and they will be brought out gradually, one at a time, Narongsak said.

A total of 18 scuba divers are involved in the effort, including 13 foreigners and five Thais.

A spokesperson from Foreign Affairs minister Julie Bishop confirmed eight Australian divers were involved.

Narongsak said it was Australian medics who also entered the cave early on Sunday to evaluate the boys' health and confirmed they were well enough to undertake the journey out.

Parts of the cave remained underwater, meaning the group would need to pass through some passages using scuba equipment.

But the water levels inside the cave have receded enough in recent days - thanks to intensive drainage efforts - that many parts can now be crossed by walking, Narongsak said.

The group of a dozen boys aged from 11-to-16 and their 25-year-old coach have been trapped inside the cave since flash floods blocked their exit on June 23.

In recent days parts of the cave were totally submerged in water. The four-kilometre route from the rocky shelf where the group has been waiting to the cave's exit has taken even professional cave divers as long as five or six hours to complete.

The perilous nature of an attempted scuba diving rescue was underscored on Friday, when a Thai former Navy SEAL diver died while bringing oxygen tanks into the labyrinthine cave complex.

Officials have also been frantically working to remove rocks and other debris from the exit path.

Water currents inside the cave in recent days were strong, visibility was non-existent in parts and the underwater route was complicated by many narrow passages and jagged rocks.

In addition, at least some of the boys are not strong swimmers.

According to a report by the ABC, as re-reported by News.com.au, the most difficult part of their rescue will be a 200-metre submerged dive, where each boy will need to cling onto a diver.

They will then have to pass the narrowest point in the cave - a 38-centimetre hole in the rock formation.

A reporter on the scene described the space as "barely bigger than a standard school ruler or the size of your head", as reported by News.com.au.

The area is so narrow, the divers will need to remove their oxygen tanks to pass through it.

It's easily the most perilous part of the whole rescue mission, with the plan to take each boy out one at a time.

"They can’t take them all out at once for fear of a dangerous bottleneck situation," said CNN’s Matt Rivers, as reported by News.com.au.