The blast, which shattered windows and blew doors off their hinges in houses hundreds of metres away, was unusually strong.
Wednesday's blast was the latest in a string of attacks in Kabul.
The bomb went off about 8.25am local time (3.55pm NZT) during rush hour in the diplomatic quarter.
This was the worst terror attack in Kabul after March 2017 when 30 people were killed after terrorists dressed as doctors stormed Sardar Daud Khan Military Hospital.
- January 10: Twin blasts near the Afghan parliament kill 38 people and wound 86. "I've never seen such a powerful explosion in my life", he said.
"Such attacks do not change our resolve in continuing to support the Afghan government in the stabilization of the country", Gabriel said.
A police officer said it was hard to say who was the intended target.
- October 12: Islamic State claim responsibility for an attack on a shrine that kills at least 18 people gathering to mark Ashura, one of the most important Shiite festivals.
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"I have been to many attacks, taken wounded people out of many blast sites, but I can say I have ever seen such a frightful attack as I saw this morning", ambulance driver Alef Ahmadzai told The Associated Press.
Australian diplomats in Kabul are in lockdown but all embassy staff are safe and have been accounted for, Department of Foreign Affairs and Trade secretary Frances Adamson said.
Among the dead were two Afghan media workers, two Afghan guards at the German Embassy and nine security guards at a base for foreign forces in the area.
Overnight a third attack, which was not claimed, targets aid group CARE International in central Kabul.
Zabihullah Mujahid, spokesman for the Taliban, says that Wednesday's explosion had "nothing to do with the Mujahedeen of Islamic Emirate", as the Taliban call themselves.
But a spokesperson said in a statement that the movement's fighters had no involvement and it condemned any such untargeted attacks that caused civilian casualties.
Massive explosion on the gate of the German mission in #Kabul.
Najib Danish, deputy spokesman for the Interior Ministry, said at least 50 people were killed or wounded in the suicide auto bombing Wednesday.
Wednesday's explosion was so heavy that more than 50 vehicles were either destroyed or damaged around the site of the attack.
The attack comes as the largely Muslim country observes Islam's holy month of Ramadan. "It's especially contemptible that these people were the target", Gabriel said.