Twenty-three people were rescued after those on board the stricken Vietnamese-listed vessel either fell or jumped into the sea, police said.
Some of the victims suffered burns, with four people injured, one of them seriously.
Fears of a potential environmental catastrophe in a busy shipping channel also known for porpoises and turtles were dampened late Tuesday after fire officials said no oil leak has yet to be detected from the vessel.
He told reporters that according to the crew on the barge, they were "connecting the hoses for refuelling for the oil tanker, and then there were three explosions", before the ship burst into flames.
Yiu added that the vessel, which was on its way from the southern Chinese industrial city of Dongguan to Thailand, was not at risk of sinking. It took around five hours for firefighters to put out the blaze.
But the tanker, which had a crew of 25 Vietnamese, was listing at a 30 degree angle.
One Singaporean national was slightly injured on the oil barge, authorities said.
Witnesses described feeling shockwaves when the explosions tore through the ship.
"I felt my boat shaking. The tremble came from the sea," said speedboat driver Michael Kwok, who told AFP he heard three explosions while out on his boat nearby.
Footage from before the fire was brought under control showed the stricken vessel listing with large plumes of black smoke coming from its middle and flames still burning on the deck.
A fireboat was seen spraying two streams of water into the sea near the tilted side of the tanker, according to an AFP reporter at the scene, with a mass of twisted metal on the deck and a charred exterior wall bearing a 'No Smoking' sign.
Three more fireboats, a helicopter and a police boat were also circling the scene.
He added that a smaller banging sound followed about 10 seconds later.
The name on the front of the tanker was Aulac Fortune, which the Hong Kong marine department tracker website showed as arriving at the South Lamma anchorage at 2.58 am Tuesday local time.
Ship-tracking websites MarineTraffic and VesselFinder both classify the ship as an "oil/chemical tanker". Local media reported that the oil tanker had already offloaded all the products it was carrying at the time of the accident, citing the marine department.
Despite no reported leakage, environmentalist Gary Stokes said the incident was at an early stage and he was still trying to assess its ecological impact, which depends on factors such as whether the tanker was fully loaded and the direction of the winds and currents.
"It is obviously something of concern when it comes to the environment with the animals out there -- it is the home of the finless porpoise," said Stokes, director of OceansAsia.org, referring to the waters near Lamma, adding its population is believed to be declining.
Turtles are also known to nest on one of Lamma's coves.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)