The Greens said it was "further proof that deep sea oil drilling just isn't worth the risk", although the party did not elaborate on what serious damage the riser could have done.
The riser fell into the moon pool, reportedly after someone pushed a lever the wrong way on the Noble Bob Douglas rig.
The platform was located with an ROV and recovered shortly afterwards with no harm to personnel or the environment.
The claim was revealed in a new report from Worksafe NZ that shows dangerous incidents in the oil and gas industry.
Among the incidents reported were fires, high pressure gas leaks and minor oil spills.
In 2013 there were 54 serious incidents, which increased last year to 61. Thirty-six of the incidents were classified as being damage to or failure of critical safety equipment, while seven were classified as fires or explosions.
"The public has a right to know about accidents like this but instead we're finding out a year later," Green Party energy spokesman Gareth Hughes said.
"Last year while the government and Anadarko were trying to convince New Zealanders how safe deep sea oil drilling is when they were drilling some of our deepest ever exploratory wells, they were actually losing crucial pieces of equipment in the ocean.
"If these overseas oil companies can't even keep track of crucial equipment while out there in rough conditions, how can we trust them to plug an oil spill over 1500 metres deep?
"The increase in dangerous incidents overall in the oil and gas industry shows that the risks of this industry are real."
He said NZ should focus on transitioning to clean, renewable energy, however the Key National government was keen to see an expansion of oil and gas exploration.
The government released its largest ever block offer last month.