A house will be the most expensive purchase you will ever make, so it's natural to have some doubts once your offer is made and the deal is as good as done.
On the one hand, you may wonder if you made the wrong decision, or possibly didn't research your options thoroughly enough. On the other hand, there could indeed be issues with the property and your concerns may be entirely reasonable. If an overwhelming sense of remorse coincides with countless web pages being trawled as you continue to look at the ones that got away, the solution to that is simple - stop looking!
To make a strong decision, with no room for doubt, the key is to plan ahead and avoid the chance of remorse altogether.
It's a great practice to document your home buying process - both before you start looking and as your search progresses. Getting ideas down around what you're looking for, what you want, and even what you don't want crystallises your vision. This enables you to refine your search to make sure you do in fact make the right choice. Once you've made the offer, you can then go back over that list and review your original intentions. It may be that the house you made an offer on does not include all of the things on your wish list, but if it covers most of them, you're probably okay to proceed.
If you're doubts are being seeded externally, the solution to that is to stop listening, start tuning into your instincts and believe in what you know for yourself to be true. One person's trash is often another person's treasure so don't let the doubts of others ruin what should be a time of celebration and renewal in your life.
When it's less about personal requirements and more about the sale process itself, then you may have legitimate cause for concern. If there are issues around the property's boundaries, title, inspections or the existing owner's willingness to be amenable to the process, then you should make your worries known. Depending on the nature of your concerns, the easiest solution would be to talk to your real estate agent before anything else. If you have questions or concerns, your agent can certainly offer some reassurance, provide a second opinion or, ideally, sort it out immediately!
As mentioned previously; a home is arguably the most expensive thing you'll ever buy so it's important to be able to distinguish concern, confusion and remorse and respond accordingly. Some post offer issues can be avoided by including specifics in the contract of sale - such as obligation for repairs to be done. For everything else - that's what your agent is for.
Just remember, while it's normal to have some doubts after buying, the Australian housing market rewards patience, usually very quickly.