What is an owner builder?

First time poster so apologies for what may be an incredibly silly/simple question.

I have purchased a home built in the 1930s - 40s that has been well maintained but does need some areas to be fixed. For example, I had to have all the electrical wiring in the house redone for safety. Additionally, I have had to have the old water heating system replaced or safety reason. Currently, our ceiling needs to be fixed. Please note, we do not intend to do any of this ourselves and have engaged with the relevant trades to do so.

Previously when we were selling our old place, we got into an issue with a conveyancer who insisted that some work we had done in the kitchen (replacing tiles and benchtop as they were cracked and broken; again by engaging a contractor) made us owner-builders and we needed the relevant permit/insurance. To us, this didn't make sense as we were simply fixing existing defects.

Can I ask the forum what constituted an owner-builder? Is this anyone who needs to engage with tradies to get things fixed in this house once the costs exceed a certain amount? Is that amount accumulative or for each specific project?

We are based in Victoria.

Thank you in advance.