The boys, guided to freedom by a diving team of 18 people that included both Thai navy SEALs and a specialist international dive team, emerged over three hours from about 4.30pm local time (7.30pm AEST).

"Today we saved four more," rescue mission chief Narongsak Osottanakorn, the former governor of the Chiang Rai province, proudly declared late on Monday night.

The successful extraction of the four leaves just five people remaining in the cave.

The operation, which began at 11am (local time) and finished by 8pm, was two hours shorter than Sunday's operation as rescue crews learned from earlier mistakes and improved efficiency.

"The team is getting used to the operation," Narongsak said. "In the cave we have more than 100 staff... for guide ropes, to fill tanks and 18 divers to get the kids."

Deputy Commander of the Third Army, Major General Chalongchai Chaiyakham, said they had "adapted the plan to sharpen it".

"I expect tomorrow will be good news," he said.

The boys were transferred by ambulance, one-by-one, to a field hospital near the cave for an immediate medical assessment before being taken to the Chiang Rai hospital by a combination of helicopters and ambulances.

They were in a better condition than the four boys rescued on Sunday, Narongsak said, adding that the boys had asked to eat padkrapao, a chicken dish with sweet basil.

The successful rescue of four more members of the team was confirmed by Thai navy SEALs, who signed off a Facebook post with the celebratory phrase: "Hooyah".

That means eight boys have been rescued with just five remaining in the cave, raising the prospect of that quintet being evacuated as soon as Tuesday -- and this incredible effort drawing to a close.

However, Narongsak cautioned that, for safety, the optimum number of people to rescue was four.

"We have still not confirmed that five will be out or not [on Tuesday]," he said. "The plan is set for four."

Rescue crews would take a 20-hour break to prepare for the next rescue mission on Tuesday.

The five people left in the cave - believed to be four boys and the coach - are said to be in good health, according to the doctor who has examined them in the cave.

The eight boys who have been rescued were not yet allowed direct contact with their families due to fear of infections and could only speak to them from a transparent glass room but Monday's rescue is a massive coup for the rescue workers and will delight a nation that has been riveted by the unfolding drama.

The second dramatic rescue operation began on Monday at 11am local time, 16 days after the team first went missing 3.2 kilometres deep inside the cave complex, while anxious relatives endured another agonising day.

Rescue workers and divers are still working in incredibly trying conditions, with roads leading to the cave thick with mud, temperatures hovering above 30 degrees for close to 18 hours a day, soaring humidity and dark clouds and thick fog hanging low over the Khun Nam Nang Norn mountain.

In a boost to the rescue effort, after raining solidly on Sunday night the heaviest rains mostly held off on Monday.

In a media briefing before the operation, Narongsak had delivered an upbeat assessment, declaring "it’s perfect conditions like yesterday", reflecting on the fact there had been fewer tropical showers than forecast.

Thai authorities have repeatedly warned that monsoon season rains could rapidly reverse the huge effort that has seen more than 100 million litres of water pumped out of the cave to clear the way for rescuers and smooth the path for the boys.

And as the press conference wrapped up on Monday night, Army commander of the 37th district, General Buncha Duriyaphan, said that he had asked Pra Pirun, the God of rain in Hinduism, "for no rain for three three days at a time, because if I ask more he might not grant it".

Thai Prime Minister Prayuth Chan-o-cha visited the cave site on Monday evening and then travelled to the hospital in Chiang Rai.

Narongsak related that the Prime Minister said of the incident: "It should be passed to all the kids... give them the information, this incident should not happen again".

Narongsak said he would not provide the names of the eight boys who have been rescued, citing concern for the families, but he did say the “perfect ones, the most ready ones” were among the first four who had been rescued - scotching suggestions that the weakest members of the team had come out first.

An official from Thailand’s Forestry Department said that despite heavy rains overnight on Sunday, “the water levels are still stable. Water is draining ok [from the cave]” and that operations to divert streams and pump out water from the cave were still under way".

In essence, the rescue plan aimed to repeat the success of the first attempt.

Thailand was united in celebrating the successful rescue of the first four boys, but the fate of the remaining nine hung in the balance throughout Monday.

Thai Interior Minister Anupong Paojinda said earlier in the day that before the operation got under way, air tanks would have to be replenished and replaced along the route.

Ambulances and a helicopter were seen buzzing around in the vicinity of the cave complex, ready to to spirit the four boys to a hospital in nearby Chiang Rai.

A huge support team of up to 90 people were involved in supporting the operation.

Earlier, Thongchai Lertwilairattanapong, a Ministry of Health doctor at the hospital where the first four boys rescued were being treated, said that reunions with their families would be delayed by 24 hours because of the need to perform exhaustive health checks and that after that, if all the tests were negative, then family could visit.

The identity of the four boys has not been publicly confirmed but Fairfax Media understands that 14-year-old Mongkol "Mark" Boonpiam was the first boy rescued.

Other media outlets have reported that Prajak "Note" Sutham, 14, Nattawut "Tle" Takamsai, 14 and Pipat Bodhi, 15, were the other three boys who had been rescued.

Australian doctor Richard Harris, an expert diver who was requested by the private British divers in the "all-star" team, and another Australian expert diver were expected to once again join the rescue mission.

Six Australian federal police divers and an Australian Navy diver were also involved, working with Chinese and American teams, to bring supplies to the third chamber in the cave, which is the launching off point for
the rescue.

Australian Foreign Minister Julie Bishop said her expectation was that four more boys would be rescued on Monday.

"If we are asked to send more of course we will, but we are part of an international response group working under the guidance of the Thai government and the Thai navy," Ms Bishop said.