Melissa Butterworth was in her Mount Clear backyard in Ballarat, Victoria, with her son Ethan Gaynor on Sunday afternoon, toilet training their new puppy.

Everything was going fine until the seven-year-old took a step near the family's clothesline, where his foot went through the soil, uncovering an unexpected drop.

Mrs Butterworth quickly grabbed her son, pulling his leg out of the ground to discover a large cavity underground.

See the video inside the hole here.

"My son and I were outside trying to toilet train our new puppy," she said.

"He took a step, like all of us have done 1000 times before and his foot just went right through the ground. I was able to grab him and help him up but it was really scary... he was in shock for around an hour afterwards. It was really just a scary situation, we could have never seen it coming."

Mrs Butterworth contacted the SES, who examined the cavity, discovering it stretched across large portions of the backyard as well as reaching some points of the home.

Having rented at the property for more than three years, Mrs Butterworth said she was shocked to discover this.

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"To find out this has been under our house for so long is really quite scary," she continued.

"Once we found out how big it is that made things even scarier. I had a sleepless night on Sunday because I just didn't know what could happen. When you think about how quickly sinkholes and mine shafts can open up, how can you sleep?"

Moving forward, significant repairs will likely have to be made to fill in the cavity, impacting both Mrs Butterworth's family and some neighboring properties.

She said the event has served as a wake up call, insisting she will now check mineshaft-maps before considering a property, and urged others to do the same.

"In an area like the Ballarat and the rest of the Goldfields, this is something everyone needs to be aware of," she said.

"We've got a property out in Scarsdale that we're now looking at doing works on to make sure any old mine shafts are filled or blocked.

"This could happen to anyone... people need to know how dangerous this could be."