"I couldn't just drop everything at that particular time but I knew it was something I wanted to talk about through pictures and sound."
He and a crew of six started work on the film in 2014 and spent a total of six or seven weeks in Raetihi at different times while making it. Their last visit was earlier this year.
Raetihi was not a wealthy town, he said. Its people were initially frustrated, then angry as the diesel spill affected their water supply. They were unable to use water from their taps for nearly two weeks.
News that skifield operator Ruapehu Alpine Lifts (RAL) was to blame leaked out slowly.
"The company sat on its hands for six days. Only when council shut off the water did they put their hand up and say, 'Yes, that could be from us'."
People got sick from using the water and children were absent from school and kohanga reo.
RAL pleaded guilty early in the ensuing court case and the size of the fine it paid was limited by law.
The town has $100,000 from that fine and is deciding how to spend it, but Mr Arahanga says that's not the real question.
"Forget the $100,000. What is Raetihi's long-term solution for water?"
He and his crew walked the Makotuku Stream from the Turoa ski area to the town. They found it had windblown rubbish from the ski area, was affected by market gardens, cattle and other stock, and decaying car bodies at Horopito.
Long term the town needs a better source of water than the Makotuku, perhaps a bore or a mineral spring on the mountain.
Mr Arahanga would like the documentary to improve processes for storing petrochemicals in every national park in New Zealand. A review after the spill showed 70 per cent of storage was substandard.
He'd also like iwi to have more say in how the Tongariro National Park, a World Heritage Site, is managed to "bring people more into the Maori way of thinking and respecting the mountain".
"Koro Ruapehu is sacred to all of us, as are the rivers and streams that flow from him."
There were some who felt there should not be ski fields on the mountain.
"In a Maori sense, below the bushline is for men. Above is for the tipua [demons, supernatural beings]."
But people were used to the ski fields and Mr Arahanga said commercial use and respect could co-exist there: "You can have 20,000 people up Mt Fuji in an orderly and respectful way, and they watch the sunrise. That's an experience."
For him, the documentary is about water in a country where people make fortunes by exporting it while nearby towns are without a pristine source. "I have used this humble little stream as an example of what's happening right across the country."
