Today, Green Party MP Denise Roche will introduce a Bill which would raise the country's quota to 1000.
The Bill has been met with support by all parties, except National, to allow it to be introduced to the house.
But Mr Key said the National Party would now oppose the Bill's introduction, 3Newsreported.
"These are really serious matters and I don't want to be flippant about these things but it is very easy for the Opposition to get up and say 'double the quota'," said Mr Key.
"Someone at the weekend was reported as saying 'bring 10,000'. With the greatest respect, do they actually understand what that would do to the system in New Zealand? It's not just that we're not a very populated country, we have to actually house people, we have to be able to give them services, we've got to be able to do all those things," he said at his post-Cabinet news conference yesterday.
The Mangere Resettlement Centre usually had intakes of 120 refugees at a time and that would lift to 150 with the Syrian intakes. There was also extra pressure on the Red Cross and areas such as translation and housing services. While New Zealand could have taken more Syrian refugees, that risked closing the door to refugees from other countries.
READ MORE: Syrian children find peace at last after horrors of war
Mr Key acknowledged community groups such as the Catholic and Anglican churches had offered to play a role in resettlement and it was possible they would be called on for refugees who knew English and could be settled easily.
However, he questioned whether some of the enthusiasm in the public to help would dissipate when Syria dropped out of the news.
"Inevitably these things are heightened when the media coverage is so intense and over time some of that might dissipate a little bit. I'm not at all questioning the churches - if they say they have capacity and want to help we'll be taking them up on that offer but practically we have to do our best to allow people to settle well in New Zealand."
Mr Key was criticised for his initial response to the crisis after he said New Zealand would not consider extra help until after a review of the refugee quota in the middle of next year. Yesterday's announcement followed an increase in public calls for New Zealand to do more.
Asked about his apparent backdown, Mr Key conceded the public response was a factor in his decision.
The situation had escalated over the past week and scenes coming out of Europe had heightened the public response to it.
Asked whether his own family history had played on his mind when making the decision, Mr Key said his mother Ruth had benefited from family reunification policies to get into the United Kingdom as an Austrian Jewish refugee.
"If she hadn't had that, there's a very strong chance she would have been persecuted and gone to the concentration camps like some of her family members did. I have a responsibility to do what's right for New Zealand and what works for New Zealand."
He said he took advice about what the country could handle before making those decisions. Many of the Syrians would likely go to Wellington where many of the 83 who came to New Zealand last year now live. That was partly because many would need Housing NZ homes and the demand in Auckland made it difficult.
Labour leader Andrew Little said Labour would now drop its bill to bring in 750 refugees over the next year, but still wanted an increase in the quota to 1000 each year and believed the Government should bring forward its review on the quota.
Green co-leader James Shaw said Mr Key's excuse that it would stretch the system did not wash.
"The system is stretched because the Government has been chronically underfunding it. It is a matter of prioritisation."
United Future leader Peter Dunne also said the quota should be increased in the longer term.
