Welcome to the Bunnings Workshop community @Ghans001. It's terrific to have you join us and many thanks for your question.
Traditionally these concrete fountains were installed onto a concrete reservoir base which acts as the pond section. If this ornament was already there previous to your occupation, please check to see if there is a concrete base/reservoir located under the soil. It is quite common for these to fill up with debris over the years and become buried.
Did you purchase the fibreglass pond shell from us as I can't find a product of similar description? A concern I have is that if it is indeed a fibreglass shell then that is a solid material and might crack if the ground is not perfectly flat and compacted. A low-density polyethylene pond could be more forgiving.
You should find this step-by-step guide helpful: How To Build A Garden Pond. Please note this is for one of our low-density polyethylene ponds and should only be used as a general guide.
I would encourage you to follow the manufacturers directions on the installation of the pond shell and to contact them to find out if it is suitable to bear the weight of a 250kg feature. Please let me know if it was purchased from Bunnings and I would be happy to provide specific installation instructions and clarify its suitability for the feature.
Let me also mention the knowledgeable @Adam_W to see if he could add to the conversation.
Please let me know if you need further assistance or had questions.
Thanks for your help Mitchell.
Bunnings have supplied the concrete, mattock, shovel, rake, wheel barrow, gloves, etc.
The pond liner brand is "gumtree" , and didn't come with instructions!
I think I will make a bed of sand (or stone dust?), then a concrete footing, then the liner.