A simple colorbond steel roof patio adjoining your built alfresco would work. You can enclose the patio and have your sliding glass doors. BUT, all of this must have approval from your local council authority. You may not be able to enclose the patio nor build as big as you would like. There will be building to open space ratios regulations that you will have to adhere to.

https://www.stratco.com.au/pavilion#slique

Thanks for the comment. At the moment, alfresco size is 3 x 3.5, if we’ll have the entension to have the current size doubled, do we still have to request for council approval?

Yes you do. The extension will need council approval. Any buildings need council approval. Sometimes if you get the professional patio people in to build your extension, they will do the council approval on your behalf.

I suggest You contact your council by phone, or go on their website, which usually has all the necessary regulations.
Regulations can change by state. So contact YOUR council, and see what is possible.