The woman, who did not want to be named, said she was just a few metres behind her boyfriend when the incident happened.
"I saw the ground collapsing under his feet and he jumped out screaming and shouting for the pain. This is something I will never forget."
She said her boyfriend was suffering from physical and psychological damage and the incident has had a bad effect on their lives and particularly their economic situation.
The couple arrived in Rotorua in early December, and the man was working in the hospitality industry.
She said they went for a walk through Puarenga Park to appreciate the environment, respecting its features, and they were extremely careful about the dangers.
"The entire area was open with no enclosures or nets."
She said they'd seen other areas in Rotorua where it was clear where they could and couldn't walk, but that wasn't the case here.
"The warning was ridiculously small given the effective risk that exists, just one very small [sign] ... about a dangerous geothermal area, nothing more."
The woman said locals might know the nature of the place but visitors and tourists didn't.
"This carelessness is a bad commercial for the city of Rotorua and right now has already cost a lot to a poor guy that has recently started to live and work honestly here."
She said they wanted honesty around the incident because they didn't want to see it happening to anyone else.
She said she didn't want to offend anybody because Rotorua was a "beautiful place", and she paid credit to the staff at Waikato Hospital for their professionalism, care and understanding of their situation.
Rotorua District Council infrastructure services group manager Andy Bell said there was signage which stated it was a geothermal area and walkers should keep to the track as well as a "DANGER Thermal Area!" sign with a no walker access symbol.
He said they believed the signage was appropriate, however, as an immediate response they had erected prominent signage to inform track users to stay on the track.
Mr Bell said there was a path alongside the stream which was elevated about 1.2m above the adjacent sinter flats which contained the thermal mud pool and fumaroles.
"There is no fencing in the area between the track and the adjacent geothermal area and this has not changed as the distinction between the track and the thermal area is quite clear."
Mr Bell said the council was "extremely sympathetic" to the injured man's situation and wished him and his partner the very best for his recovery.