But 63.3 per cent said they should be withdrawn as scheduled, and 13.6 per cent said they did not know.
The SAS troops are based in Kabul and have been training and mentoring the Crisis Response Unit, an Afghani force that responds to incidents including terrorist attacks.
In recent attacks, the fighting has escalated and the SAS has stepped in.
Last month the SAS intervened in a three-hour gunfight with Taleban insurgents who had stormed the home of Jan Mohammed Khan, a close adviser to President Hamid Karzai.
The previous month, it played an integral part in ending a night-long assault by nine insurgents on the Inter-Continental Hotel in Kabul.
The Labour and Green parties oppose the SAS deployment because they believe the troops are helping a corrupt regime that does not have the trust and confidence of its people.
The SAS did three six-month tours of duty in Afghanistan between 2001 until 2005.
They returned in 2009 and the latest posting was extended to March, although the number of soldiers involved was halved.
The SAS had wanted to stay on, and Mr Key has said some soldiers had since expressed a wish to continue beyond March.
But Mr Key has said the SAS were not US guns for hire.
"I made that decision to send the SAS back to Afghanistan. I made it with my eyes open, and I made it very clearly and deliberately, and it was because I believed that New Zealand had to demonstrate that it was taking its responsibilities seriously as it came to being global citizens."
New Zealand also has a provincial reconstruction team in Afghanistan, based in Bamiyan province.
It will be gradually withdrawn between now and 2014 as transition of power to local authorities takes place.
THE NUMBERS
Do you think the New Zealand SAS should remain in Afghanistan beyond March next year or be withdrawn as scheduled?
* Should remain beyond March 23.1%
* Should be withdrawn as scheduled 63.3%
* Don't Know 13.6%
Sample size: 750, margin of error 3.6%