Could my downpipes be going into the sewer system?

I recently moved into a house in NSW built in the mid 90s and I was curious about how the stormwater drainage works, specifically, could some of the downpipes from the gutters be directed to the sewer system?

Here is why I think it might be the case:

* I recently tested two downpipes on the north side of the house and neither of them seemed to cause water to come out of my street outlet, which I'm certain is the correct outlet (none others in our boundary). However we had a lot of rain recently (500+ml over 3 days) after I tested them, and the street outlet performed well removing water from the property. During this rain event, the two north side downpipes I tested seemed fine - no backlogging, and they sounded like they were draining. So they seemed to be draining somewhere.

* Also recently, a weird thing happened at night - I noticed one of these downpipes was making a strange bubbling sound. This was a few days after the big rain, but it didn't seem blocked then or now from what I can tell, so not sure what the bubbling was. If it was going into the sewer pipe, it could have been from there? or if it has a U-bend, something from the sewer like air pressure etc causing it to bubble?

* The previous owner did a bathroom reno and left some plans, including original plans for the house. This has no stormwater plans unfortunately, but it does have sewer and water plans. The sewer pipe does come up the north side right by these two downpipes, and on the plan, at least one of them seems to have a circle (tie in point?) near a downpipe with nothing else nearby that I could imagine needs it (this downpipe is well past the bathroom, also along the north side of the house).

Anyway, as far as I'm aware this would be illegal now and probably was in the mid 90s when the house was built as well. I'm assuming it is very unlikely it is the case, but if it did turn out the north side downpipes are connected to sewer for some reason, could I be liable to fix this if it is found out? like what if I need a plumber to come snake a downpipe and they realise it, would they have to report it?