The existing light is smaller than the size of the heat fan light. (I/N: 4442496) The cut out is needed.
Welcome to the Bunnings Workshop community @Hui. It's terrific to have you join us and many thanks for your question on installing a heat lamp.
Great question.
We do offer an exhaust fan and heat lamp installation service, which starts from $160. Since your existing cut-out is smaller than the new unit, the installer may need to enlarge the opening or do some additional work to fit it properly.
Any extra costs can be discussed directly with the installer on the day, as it will depend on what’s required once they assess your setup.
If you’d like, you can arrange this through your local store or online, and they’ll be able to guide you through the next steps.
Please let me know if you have any questions.
Mitchell
Thanks @MitchellMc