The skipper and navigation officer have been jailed for seven months each after their catalogue of blunders led to the grounding of the cargo ship Rena and New Zealand's worst maritime environmental disaster.

Captain Mauro Balomaga, 44, and navigation officer Leonil Relon, 37, both Filipinos, were sentenced in Tauranga District Court on Friday on a raft of charges laid after the 236m, 37,000-tonne cargo ship struck Astrolabe Reef off Tauranga in the early hours of October 5 last year.

It spilled about 360 tonnes of heavy fuel oil into the sea, which washed up on local beaches and killed wildlife. Containers were washed overboard and clean-up crews are still picking up debris from the wreck, which broke apart in January after being pounded by heavy seas.

The disaster, which sparked a massive anti-pollution response, ruined the summer for many in the Bay of Plenty as beaches were spoiled and tourism suffered.

Both men, whose name suppression was lifted on Friday, had earlier pleaded guilty to wilfully attempting to pervert the course of justice by altering the ship's documents after it grounded.

They had been facing maximum penalties of seven years in prison and a $300,000 fine.

They escaped any fine.

The court was told the crew were trying to get into Tauranga Harbour as quickly as they could and broke the "basic principals of navigation".

In trying to take a shortcut they diverted course but the change was not noted on the chart and no effort was made to project the ship's course.

About 10 minutes before the ship grounded, the Astrolabe Reef appeared on radar, but Balomaga believed it was a small vessel or a false echo, and he took no evasive action.

Crown prosecutor Rob Ronayne said each man's actions displayed "incompetence verging on recklessness".

After the grounding Balomaga instructed Relon to alter the charts, computer and GPS log to make it appear the Rena was planning to pass clear of the reef.

Maritime New Zealand director Keith Manch, in a statement welcoming the sentences, said that had caused genuine confusion for investigators trying to piece together events.

"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."

Mr Manch said the sentencing was a milestone in the ongoing clean up effort, which was earlier this year estimated to have cost $130 million.

More than 800 of the 1368 containers aboard the ship have so far been recovered and most of the 1700 tonnes of heavy fuel pumped off.

The ship's owner, Greece-based Daina Shipping, has also been charged with discharging harmful substances and its case was put off until July.