The owners of the Rena should share the blame for the disaster that led to two Filipino seamen being jailed, says a Philippines-based seafarers group.

Captain Mauro Balomaga, 44, and navigation officer Leonil Relon, 37, were jailed for seven months each on Friday on a raft of charges laid after the 37,000-tonne Liberian-flagged cargo ship struck Astrolabe Reef off Tauranga in October last year.

In what became New Zealand's worst environmental disaster, the Rena 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.

Investigators should look into the possibility the ship owners had ordered Balomaga to alter course to save fuel, Edwin de la Cruz, president of the Manila advocacy group International Seafarers' Action Centre, told AFP.

"The captain should not be made a scapegoat... the act of the captain is the act of shipowners."

The ship's owner, Greece-based Daina Shipping, has been charged with discharging harmful substances and its case was put off until July. Daina is a subsidiary of Greek ship charter company Costamare.

Mediterranean Shipping Company, which chartered the vessel, donated $1 million toward the clean-up.

Maritime Union general secretary Joe Fleetwood agreed with the suggestion the disaster was not purely the fault of the two men.

New Zealand had deregulated its shipping industry and now relied on international lines - many using flags of convenience - for its coastal cargo system, he told NZ Newswire.

He did not think New Zealand would return to the days of having its own ships serving its ports, and predicted there would only be a "band-aid" response to prevent a disaster like the Rena happening again.

Maritime New Zealand and the Transport Accident Investigation Commission (TAIC) are both investigating the grounding.