Mr Frentz said the commissioners decided there were potentially significant adverse effects resulting from contaminants on the reef's ecology and fisheries, human health and the natural character, landscape and Maori values.
He said two further rounds of reef monitoring had allowed a comparison to be made between the effects of leaving the Rena in a small area of the reef and the reef's wider habitat.
It had demonstrated that TBT, a banned biocide used in anti-fouling paint, was dispersing, so its intensity would diminish. The other contaminant with potential ongoing environmental effects, the 12 tonnes of copper clove believed to be in the collapsed remains of a cargo hold, was localised and "unlikely to disperse further", he said.
Mr Frentz said there was a very low probability the effects of leaving the wreck would be greater than the effects that had already occurred from the grounding on October 5, 2011.
He agreed with the commissioners' decision that the wreck would have a negligible effect on the abundance and diversity of reef ecology, and that the ecology would recover naturally in the limited areas affected by the grounding.
"Similarly, the monitoring has shown that the contaminants released to the environment do not pose a current risk to human health. Monitoring since 2013 has shown only limited presence of contaminants in fish species and there is no apparent trend in that contamination."
He agreed with Dr Francesca Kelly that there was no increased risk of toxicological health risks if the wreck was left in place.
"This remains true even for high consumers of seafood and for those who have greater susceptibility to adverse health effects due to individual health risk factors."
Mr Frentz said a pragmatic real-world view was that the site was now in a state where further effects were "a low probability". Any effects that may occur were localised and less than high potential impact.
Maori who had withdrawn their appeals or supported the application recognised that without consent there would be no restoration or mitigation conditions and no opportunity for future input to the management of the reef.
Under cross-examination by Tom Bennion representing the appellants, Mr Frentz said about 10,500 tonnes of steel of the ship's original 14,500 tonnes remained on the reef.
Mr Bennion asked whether there was a "natural tension" between allowing nature to recolonise the wreck site and that the wreck could move in a big storm.
Mr Frentz said the evidence from monitoring was that future movement was not likely to a significant degree.
Key dates in Rena wreck process
February 2016: Consent granted to leave wreck on reef
March 2016: Seven appeals lodged to decision
May 2016: Court-assisted median on appeals
March 6 2017: Environment Court begins hearing two remaining appeals
