The intruder had turned on both the showers in the flat and left the doors open.
The extent of the damage had not yet been assessed, but water leaked from the upstairs bathroom into the ceiling of the kitchen.
He had no idea why someone would do that. "What type of mindset do you have to be in?"
Four residents were home and asleep at the time of the break-in, about 1am.
When he arrived home at 3am, Harrison said he thought it was strange his flatmate was having a late shower, but just went to bed.
He and his flatmates had left the back door unlocked, which was how the intruder got in.
After talking with neighbours they learned the burglar had attempted to break into other flats as well.
One neighbour had CCTV footage of a person trying to enter a nearby property at night.
Senior Sergeant Craig Dinnissen said police were investigating.
Harrison said he was not too shaken by the break-in.
"If it was more malicious I'd probably feel less safe. It's hard to say it's not kind of comedic.
"Obviously, we will be locking our doors from now on."