(CNN) -- The captain and second officer of a container ship that ran aground last year, resulting in New Zealand's worst maritime environmental disaster, have been jailed for seven months.
Maritime New Zealand (MNZ) said in a statement Friday that skipper Mauro Balomaga and Navigator Leonil Relon -- both Philippines nationals -- were found guilty of a total of 11 offences, including operating a vessel in a manner causing unnecessary danger or risk, discharging harmful substances, and willfully attempting to pervert the course of justice by altering ship's documents subsequent to the grounding.
The cargo vessel, Rena, hit a reef off Tauranga in clear conditions on October 5 last year. At least 350 tons of fuel oil spewed into the sea, killing thousands of sea birds and contaminating the pristine beaches of Tauranga's Bay of Plenty.
After a passage plan for the vessel's journey had been plotted, the captain gave approval for the second officer to deviate from the plan to save time, MNZ said. The subsequent changes to the ship's course, which were not verified using appropriate navigational methods, put the vessel directly on target to hit the Astrolabe Reef.
About 10 minutes before the grounding, the reef appeared as an echo on Rena's radar. At this stage, there was still sufficient time to make an effective alteration of course and avoid the reef, the MNZ statement said.
The captain saw the echo and assumed it was a small vessel -- but when he could not find it, he dismissed it as a false echo.
These two senior officers, who were responsible for the safe navigation of the ship, had failed in their duty, MNZ director Keith Manch said.
"In this case, the master and the second officer have breached the most basic fundamental principles of safe navigation," he said.
"If there is a need to deviate from a prepared passage plan, the alterations must be plotted and the new projected path carefully assessed to ensure all potential navigational hazards are identified."
Manch added that both officers also admitted making alterations after the grounding to the ship's GPS log, its passage plan and its computer to mislead investigating authorities.
"This offending is also very serious in that it caused genuine confusion for investigators trying to piece together the events that led to the grounding," he added.
"It is vital that when these types of events do take place, we can find out how and why they have happened to help prevent such an event happening again."
Earlier this week, MNZ said "excellent progress" has been made on container removal from the wreck site over the past week, with 21 containers removed from the bow section of the vessel on Monday -- the highest number of containers removed in a single day since the break up of the wreck. It said 815 of the 1,368 containers on board have now been recovered.
It added that tons of debris, such as timber and plastic continues to be removed from the Bay of Plenty coastline on New Zealand's North Island.