The Bundaberg resident was on a month-long fact-gathering mission of birdlife and other marine animals.
The West Atlas rig and Montara well head platform, located more than 200km off the Kimberley coastline in Western Australia, began to leak oil on August 21.
The slick flowed with the tide towards the reef.
The rig then burst into flames last week as the company that owns it attempted to block the leak with mud.
Ashmore Reef is heavily controlled by Australian customs and few people are allowed to venture there because of the fragile nature of the environment.
Mr Moorhead left from Broome on October 5, travelling to within 140km of Indonesia.
“It’s light crude oil (that spilled) and it has had minimal environmental damage,” he said.
“The slick in the photographs — we’re not sure if it was dead plankton or wax from the retardant.”
Mr Moorhead said the retardant was what the company that owns the rig had put on the oil in an attempt to mop it up.
“Because it was light crude, it may have evaporated once it hit the surface,” he said.
“The retardant may have actually made it worse.
“They spent a lot of money and could have made (the spill) worse.”
The Bundaberg conservationist said while it was a significant spill, it was still a long way from breeding bird colonies on the reef’s many islands.
“But a smaller spill closer to the islands would have been a disaster,” Mr Moorhead said.
The environmentalist said it was a warning not to “muck around” in pristine conservation areas.
“We’ve dodged a bullet on this one,” he said.
“We have to be careful about mining.
“It’s often impossible to juggle environmental needs with the mighty dollar.”
Mr Moorhead spent the month with his son, Jack, and some of the top birdwatchers in the country observing and recording the different birds, turtles, and other marine life that used the islands in the reef, or passed through during their migration.
“It’s incredibly hot and it’s long days,” he said.
“We’d start at 4.30am, have a snooze at lunch and then work until about 6.30pm.”