Barrister Rita Nabney said the staff and two tour guides were the only people on the island at the time of an 8am tour on Saturday, and spurting mud reached about 15 metres high.
At the time, the sights left them in awe after a cloud covering the crater lake blew over to reveal the active volcano.
Watching the mud spurting from the edge of the crater, wearing gas masks and hard-hats, the thought of an eruption, or any other danger, never crossed her mind.
"To be frank, I never would have thought this would happen," she said.
"It's a great tragedy ... it's just so sad."
She said one of the greatest shocks was seeing how quickly things could change from an acceptable risk.
There was no life on the island, and an eerie wind blew as they walked and the only noise was the "popping and bubbling" of the volcano.
The guides told them the volcano could erupt and they needed to run for cover to a shipping container which had food in it, should they need to.
The volcano was an active Level 2 at the time and Toni Nickalls who was also attending the staff function said it was erupting, on a small scale, and "shooting up like a geyser".
"I must admit, when we were there I did think - if it did blow now we wouldn't have a shot.''
There was a shock that went through the office on Monday, she said.
"We were just talking and reminiscing and said, hell, that was a bit close for comfort," Nickalls said.
Nickalls grew up in the area and initially thought the volcano was "just going off again".
"We were very fortunate it didn't happen that day, those poor people."
Nickalls said she hoped the tours would not be stopped completely.
