Hi @karapoti,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
From a regulatory point of view, there is usually no approval required for adding insulation to an existing roof.
You can definitely add insulation under an existing tin roof like yours, and it will make a noticeable difference to heat buildup. Metal roofs heat up quickly in the morning and hold that heat for hours if there is no thermal break underneath.
The best solution for a retrofit is a foil-faced metal roof insulation blanket like this Earthwool Space Blanket, which is produced by Knauf Insulation. These products are designed specifically to sit directly under tin roofing and combine bulk insulation with a reflective foil layer. The foil reflects radiant heat away before it enters the roof space, while the bulk insulation slows down heat transfer.
This Knauf Space Blanket Installation Guide is the document you'll need to check for guidance when installing.
Typically, it would be laid before any roofing goes down, but if you plan things out, you shouldn't have any issue retrofitting it while removing one or two roofing sheets at a time. The main thing is that the foil faces down into the ceiling cavity, and the insulation faces up and forms to the contours of the roofing sheet.
Before installing the insulation, you should check that cables are in good condition and not tightly wrapped or compressed by the blanket. If anything looks old, damaged or concerning, the instructions clearly indicate that an electrician should be involved in the installation process.
Let me know what you think and if you have any further questions, please don't hesitate to ask.
Jacob
A note for our New Zealand readers - The use of foil faced insulation products has been banned in New Zealand
