A man, later shot dead by police, had ploughed a car into pedestrians on Westminster Bridge before stabbing a police officer to death inside the gates of Parliament. Despite the horror outside Sir Paul was not afraid.
As a member of the joint House of Commons and House of Lords security committee, which helps make decisions about security at the home of both houses of Parliament, he was privy to knowledge of how they were being kept safe.
"It's a very secure and safe feeling to know if something does get nasty like that ... you've got people who can deal with that."
Those inside the House of Commons were later escorted to the adjacent Westminster Abbey, where they were looked after with cups of tea and other hospitality before it was deemed safe to leave two or three hours later.
One part of the tragedy had been playing on his mind: The death of the unarmed police officer, Keith Palmer, who rushed to confront the knife-wielding man and paid with his life.
Sir Paul knew Palmer, a husband and father, and he also knew the security committee had been considering making a change at the gates, which may have prevented the assailant coming through.
"Perhaps if we had got the gate, this guy, he might not have gotten up [there]."
Asked if Palmer was a hero, Sir Paul said all of the police were.
"They will face up to it. They really are the thin blue line."
The work of the security committee will go on, with one concern how to deal with drones, he said.
"Some of the bigger ones can carry bombs ... it's a dangerous place when you consider over the last three years the police have stopped something like 11 or 13 threats.
"But tomorrow at the usual time the House of Commons and the House of Lords will open and we will not let the terrorists win."
Other Kiwis also described their experiences as the dramatic day unfolded.
Auckland woman Natassia Snell, 25, works around the corner from the Westminster Bridge at a hedge fund firm.
They heard the news almost immediately and their building was put into lock down, she said.
"There are lots of sirens going on and they've closed off some streets and redirected all the buses."
Snell thought the tube stations would be "crazy ridiculously" busy as people bustled to get home but instead found her station to be quiet
"Normally my tube stop is packed but I got a seat on the first train. I never get a seat. If something bad happens no one takes public transport."
Snell usually tries not to think about what can go wrong in London.
"You know London is a red level terrorist alert. But if you did think about it, how would you do anything?"
Kiwi Juliet Brine works for an investment company near Embankment, about a kilometre from Parliament.
She and her colleagues were in their offices when they heard "a lot of commotion" and saw helicopters and emergency responders, but didn't know what was going on.
"The whole rest of the day it was just helicopters and helicopters, and the helicopters in London are so big."
Brine took the tube home and every station was "swarmed with police".
She felt rattled that the tragedy had occurred so close to her work, but was reassured by the reaction of those in the city she had called home for almost two years.
"Everyone has been so calm. Typical Londoners."
