"It's not the case that nothing was being done," he said.
Mr Manch said the organisation was so focused on its response to the grounding and spill, it failed to let people know what it was doing.
"We had 200 people in a very short period of time who needed to respond. We had to focus on administrative issues like finance, IT and HR ... we can do better next time," Mr Manch said.
When asked if Maritime New Zealand (MNZ) would be ready to handle another Rena, Mr Manch said: "Absolutely".
Yesterday, the Government announced $2 million would be invested to improve New Zealand's maritime response capabilities. Its order for full wreck removal remained in place, but Mr Manch understood the salvage efforts had been delayed because of weather.
University of Waikato Chair in Coastal Science Professor Chris Battershill said there was minimal lasting environmental impact except for heightened levels of contaminants contained to Astrolabe Reef and the north-east area of Motiti Island, which were still being monitored.
Professor Battershill said Rena's minimal impact, compared to Australian spills with similar oil tonnage, was a "pleasant surprise" despite the marine environment still not reaching a pre-Rena state. While he confirmed small amounts of oil and tarballs were washing ashore, they did not present a major concern.
In other findings, juvenile finfish such as kingfish were found to have higher exposures to heavy fuel oil and oil dispersant Corexit.
However, because Corexit was used briefly at sea, strong offshore wind conditions at the time meant there were no environmental effects, he said.
Two years ago people were crying out for research on the environmental effects from the use of dispersant such as Corexit, but at the time there was very little research to offer, he said.
"Now we've learned from that. That's the legacy of Rena - that information." Tauranga Moana Iwi Leaders Forum chair Awanui Black agreed MNZ had failed to engage with the community and iwi in the immediate aftermath of the grounding.
He was concerned the positive environmental findings would weaken the order of full wreck removal. "I'm very concerned that's the case. It's the reason behind leaving it there, that's the concern. Not necessarily because it's a good thing for the environment although that's what it might be painted as."
Mr Black said iwi believed the proposal to leave the wreck on the reef was solely aimed at saving money.
Motiti Island spokeswoman Rangi Butler said she felt the reports were just a public relations exercise and she was not happy with the findings.
"I was out there last week. I could smell something was not right with the kina. I could still smell it there," Ms Butler said.
The contaminants had robbed the islanders of their pantry, she said.
"It's out of sight, not out of mind.
"We know it's there. We know it should not be there.
"We also know it should be removed, concerning what's down there at the moment.
"The owners have the money to remove it, so don't leave it for us to remove it.
"It's not our accident. It's theirs.
"They should be responsible," she said.
Monitoring of more long-term marine effects will continue, with findings expected next year.
