More than 300 tonnes of toxic fuel oil spewed from the vessel, creating an oil slick kilometres (miles) long, which washed onto beaches at the popular tourist spot, coating birds in thick black sludge.
Environment Minister Nick Smith described it as New Zealand's worst maritime pollution disaster and claimed shortly after the accident that the Rena hit the reef while taking a short cut to reach port.
The disaster triggered a dangerous salvage operation which involved crews scrambling to pump remaining oil from the Rena's fuel tanks as heavy seas pounded the stricken vessel, opening up deep cracks in its hull.
An army of 5,000 volunteers was also mobilised to clean up the shoreline of the bay, which contains marine reserves and teems with wildlife including whales, dolphins, penguins, seals and rare sea birds.
MNZ said this week that clean-up teams had removed more than a cubic kilometre (0.24 cubic miles) of waste such as polluted sand and soil from the shoreline.
The vessel eventually broke up on the reef in January, when the stern sank, further complicating a salvage operation which is still continuing after five months as crews remove shipping containers from the bow.
Earlier this month, Smith estimated the disaster clean-up cost would total NZ$130 million ($110 million), most of which would be covered by its owner, the Greece-based Costamare Shipping Company,
Prime Minister John Key said the guilty pleas vindicated the charges.
"It's important justice (is) bought to bear here," he told Fairfax Media.
"Significant environmental damage (has) occurred in New Zealand and the government is very concerned about that."
In total, the captain pleaded guilty to four criminal charges and the second officer to three. Each of the charges carries a maximum penalty of seven years' jail.
The charge of operating a ship in a dangerous manner can attract a one-year jail term, while the maximum sentence for discharging harmful substances is two years or a NZ$300,000 ($252,000) fine.