De Kock was testifying in support of his application for amnesty for the attacks on the Johannesburg offices of the Congress of SA Trade Unions in March 1987 and the headquarters of the SA Council of Churches in August 1988.
He described the missions to destroy the buildings as being extremely hazardous and said they had required careful planning.
He said carrying high powered explosives in built up areas put many innocent people's lives at risk. He also knew that if the missions failed and the bombers were apprehended, it would have caused great embarrassment to the government.
"It would then be revealed that the security forces had turned to state-sanctioned terrorism," De Kock told the committee.
He began his testimony with a detailed account of how he prepared for the Cosatu House operation after being told to carry it out by Brigadier Willem Schoon, who told him the order had come from highest authority in government. He said he assumed this to be then president PW Botha.
De Kock said he arranged the purchase of torches, short stabbing knives, a length of rope and bolt cutters for the mission. He also arranged firearms, including AK47 rifles which were tested and fitted with silencers.
Cosatu House was then placed under observation from the rooftop of a nearby building and a video recording of the inside was made with a camera concealed in a briefcase.
He said the explosive charges were prepared in conjuction with the SAP's explosives unit. Explosives of Russian origin were used so that it would appear they had been stored in the building and had been detonated. He estimated that about 50kg of explosives were needed to wreck buildings.
When the bombing party set off from a safe house in Honeydew outside Johannesburg, they removed everything that could identity them as policemen. They also took a few cans of beer that had been doctored with drugs which were intended to incapacitate guards at the building.
The bars to the building's basement were cut and the explosives experts climbed down a rope to set the charges.
De Kock said the whole operation took about four minutes and after leaving, the team watched the explosion from a highway east of Johannesburg.
Referring to the Khotso House blast, he said he was asked to carry out the bombing by former police general Gerrit Erasmus, who he premused had received the order from the top level of government. Again he assumed it came from Botha.
The planning for the attack took a similar form but he said the operation was almost torpedoed when a black uniformed policeman looked into the basement from the street above. De Kock said he believed the policemen might have seen something untoward so he decided to expedite the operation.
The explosives which had been carried in backpacks were quickly placed at the entrance to the lifts in the basement and the detonators set.
He said the team left the scene and waited near Hillbrow to watch the explosion. He said if the bomb had not detonated they would have had to return to reset it.
De Kock was asked about claims that he had made in the past that Erasmus had told him to shoot any policemen who might have come across the bombing party.
De Kock confirmed that Erasmus had said those words and he had taken them literally.
Erasmus testified to the committee last week that he could not remember giving that order, but said if he had, he had not meant it.
De Kock on Wednesday said he accepted that explanation. He added that he might only have considered shooting other policemen in an "absolutely extreme" situation.
De Kock said he wished to correct the impression "once and for all" that the security police group at Vlakplaas was a rogue unit.
"If it had been a rogue unit, we would have built an atomic bomb, and probably used it too," he said, adding that the members of Vlakplaas were dedicated policemen who worked under extreme pressure.
De Kock also described former law and order minister Adriaan Vlok and his former deputy Leon Wessels as the only honest National Party politicians.
When it was pointed out that former NP politician Roelf Meyer had also testified to the TRC, De Kock replied: "Ill reserve my judgment on Roelf Meyer."
Vlok last week accepted full responsiblity for the actions of the police members who served und the security police, Commissioner Johan van der Merwe, are also applying for amnesty for the Khotso House and Cosatu House bombings.
The hearing continues on Thursday.