One Nation senators Malcolm Roberts, who has long argued the case for global warming doesn't stack up, and Brian Burston were also on the reef trip.
Notably absent is One Nation senator Rod Culleton, who's currently on the outs with Hanson. He is also facing a High Court inquiry into whether he is eligible to remain in Parliament because of a larceny conviction - later annulled - at the time of the election.
Roberts said people had stopped coming to the reef because they were being told it was dead and that Australia should not be reporting on its health to the UN agency UNESCO.
An invitation for media to join the senators for a party meeting on the reef, sent by a spokesperson for Roberts, read: "You can't miss senator Hanson and her colleagues getting in a wet suit! This will be the most Aussie footage of the year.
"We wish to highlight to the world that the Great Barrier Reef is alive and well. We have feedback from international bodies and local tourist operators that there has been a sharp down turn [sic] in tourism because various groups have lied and declared the GBR 'dead'."
Conservationists are concerned climate change is putting severe stress on the reef, which experienced a massive coral bleaching event this year, and some have declared it's dying at an unprecedented rate. It is the most severe coral bleaching event on record.
According to conservationists, Hanson and her senators visited the wrong part of the reef as the southern sections had been the least affected by the worst bleaching.
The World Wildlife Fund said One Nation should have visited Lizard Island where bleaching, caused by high water temperatures, has killed much of the coral.
- With Megan Palin