Please seek professional advice on what posts to use and how they are installed as the force on a shade sail can be significant. In the Bunnings guide, the installation of the three steel posts was overseen by a qualified builder to ensure they were structurally sound. - Jason
Shade sails are interesting things. There is actually a lot more engineering involved in installing them that many people realise and the bigger they are the greater the requirements.
Being an actual sail, albeit a porous one, they can be impacted by an enormous amount of uplift which has the capacity to pull poorly installed or inadequate posts from the ground. In extreme wind these can then be whipped around doing enormous amounts of damage.
When I did some research with the local council I found that up to a certain size is allowed without any sort of permit but they must be installed correctly. However they also had a clause which said that if attached to a structure - house, garage, etc. - then full planning & engineering approval was needed. A shade-sail in strong wind can tear down the top courses of a brick wall of rip off fascia/barge boards if not properly installed.
Check your council web-page for 'exempt & complying' development rules. Last thing you want is council jumping on you after you've installed.
Steel posts will give you the most substantial points to fix to but a wee bit difficult to advise what type of post as there are a lot of variables.
You can use a post or a pole, your choice, as the strength is more about the diameter and wall thickness. I would be looking at a minimum diameter of 90mm (for a square post) with a wall thickness of 4mm however... the taller the post and the more exposed the position the more heavy-duty your post may need to be.
When it comes to fixing your posts down they can either be concreted in-ground - footing size will be determined by soil type and conditions, I'd be looking at around 400 to 500mm wide by at least 600mm deep - or you can buy posts with feet attached and these can then be attached to a concrete slab or suitable footing with concrete bolts.
It might even be worth seeking some professional advice. Shade sails are one of those projects that aren't overly difficult but do need to be done properly. - Adam_W
I am in the middle of the process to build a deck and shade sail over it. Plan was to install the posts that will support the deck and long enough to install shade sail on it as well. I had an engineer visit for site inspection to draw the plans. He said if I want to install shade sail I'll have to do strip footing. Else normal post holes are enough for the deck. - dharmesh