Shark said she and her team were "fine" after the incident and were not threatened by the two people on the bus.
Both migrants left the bus after being discovered, and Shark told the Australian Broadcasting Corporation on Tuesday that "they didn't threaten us or say a word - they just walked away when found".
"Scary. We just found two people hiding in our tour bus, in the luggage carrier, trying to sneak across the border into the United Kingdom", she said.
In a Twitter post to her 24,000 followers following her performance in Belgium on Sunday, wrote: 'Scary.
- Amy Shark (@AmySharkMusic) January 27, 2019The situation is scary.
"No idea how they got in there or who they are or their plans".
Shark, whose real name Amy Louise Billings, is best known for the hit songs I Said Hi and Adore.
The Love Monster singer is now touring Europe, and is due to play the first of three shows in the United Kingdom tomorrow.
She said it had given her crew a scare, but that she felt for those choosing a "dangerous way to chase a new life".
More than 12,000 migrants attempted to stow away on lorries bound to the United Kingdom in 2017, the French interior ministry reported.
There's been a spike in migrants attempting to cross the English Channel from northern France in recent months, with 129 people - primarily from Iran and Syria - trying to enter the United Kingdom in December, according to The Guardian.