He later challenged critics to say what could have been done better.
"Show me how you'd go faster? Show me how you'd do anything different? You'd mobilise the best people in the world, work out exactly what the structural damage was, how to get the oil off the ship, which barge to put it in ... that's exactly what's happened in the first four days," he told TV3's Campbell Live.
Labour leader Phil Goff, who yesterday made his first visit to the area since the accident, said the response was not good enough.
"If we can't respond to a ship that we knew right from the start was going to be in real trouble in four days of fine weather, then there is something fundamentally wrong with our ability to respond to a maritime disaster."
But he tempered his comments by indicating the problem might have been the response plan, rather than the execution.
When asked to pinpoint exactly what the Government should have done, he said: "I'm not an expert in this area. But most New Zealanders feel, whether it's systemic reasons or specific reasons for this incident, that if we can't deal with a disaster in a four-and-a-half-day window of opportunity, then we've got to change our system."
The Green Party, whose MPs have been in Tauranga since Saturday, said the Government should have taken control of the salvage operation sooner.
Green MP Gareth Hughes said the Government should have been pumping oil from the Rena from the first day.
"The fine weather was squandered. Maritime NZ should have been planning for the worst possible scenario.
"On Friday [it was reported] the Awanuia hadn't even been requested ... the navy wasn't mobilised till late in the piece."
He questioned whether New Zealand had the right equipment for the job.
A Maritime NZ report in February on its plan for dealing with oil pollution found no significant gaps in the response plan.
Mr Key said whoever was responsible for the disaster would be held to account, but the ship company's insurance liability was capped and it was possible the taxpayer would have to pick up the rest of the bill.