How will my current income be effected if I register an ABN and begin running a small business on the side?

It all depends if you are making a profit with your business. If there is an accounting profit it will be added to your wages, and you will have to pay tax at the appropriate tax rate. You should budget for this during the year so you don't get caught out at tax time.

If your business is making an accounting loss, i would assume based on what you've said that you would not be able to offset this against your income, but instead it would carry forward to offset any future profits. In this case your tax situation would be similar to how it is currently. There are some situations where you can offset losses against your wages, but you would need to speak to your accountant about your specific circumstances.

Speak to your accountant before starting the business or getting your ABN, as by setting the business up a certain way, you may be able to save more tax. Also check if you need to register for GST as this may mean some extra work for you, completing BAS's etc.

Good luck with your venture.