"We were at Downing St a couple of days before the Prime Minister stood in the same spot and announced his resignation. It was a massive decision and it was interesting to be a fly on the wall," Mr Olsen said.
The day of the referendum was the day Mr Olsen received his Queen's Young Leader award at Buckingham Palace from the monarch herself, an encounter which lasted about 30 seconds.
"I said, thank you for making sure young people are recognised and making sure they have their voices heard ... She said thank you very much for raising that and congratulations on your work and go forth and do more - that was the gist of it," Mr Olsen said.
"It was pretty surreal ... It was special and put the responsibility on to keep going."
Mr Olsen has spent four years on the Whangarei District Council youth advisory group, is a youth ambassador for Unicef NZ, and has represented New Zealand as a youth delegate.
At 17 he helped set up a free healthcare clinic for youth in Whangarei. He also works with the national youth advisory group, advising government departments and NGOs about issues ranging from mental health to education.
The Young Leader Awards recognised 60 young people from Commonwealth countries this year and included an 11-day residential programme in the UK.
Mr Olsen said it was a fascinating time to be in London.
"London voted to remain. So that was interesting, they were on the losing side. People were serious and worried about what was going to come next," Mr Olsen said.
"The other thing I noted was some of the awful xenophobic language ... I would really hope that I wouldn't see anything like that in New Zealand in the volumes I saw in London. But then you have to remember it was a knee-jerk reaction from a select few."
Mr Olsen said although "it sounds cheesy" the best part of the trip was meeting other like-minded young people. "They were from 45 different countries and all doing very different work but you can take the best and mould it into somewhere like New Zealand. Yes, you definitely don't get to go to the BBC or Downing St every day but meeting those others and hearing their stories was really special."