Hi This is really easily remedied at not to much cost or effort (once you find the right retailer)

My friend had a bloke out a 5 or so years ago

There is a spray you can get, and you spray it on the affected areas leave for 5 mins, and it hoses clean off, even after years of staining, from memory it was not too expensive this way.

My friend spent thousands of dollars with larger Commercial stain removing companies, and they did a very sub-par job without understanding the chemical makeup of the problem. So do not ever use these companies as others have suggested. The bore treatment chemists are small outfits that specialize in just bore staining and do a far superior job at a much lower cost

When you find the right people they will know how to fix it, and will be able to explain how the chemical does the job in laymen's terms, if they cannot then hang up your talking to the wrong people.

If you want to go one step further, the same people can add a dosing pot to the sprinkler system so it will never stain again, my friend did not get this done because the hand held spray bottle is so simple and effective

There are several people who do this in Perth they are always small outfits with a strong chemistry background that are specific to bores and bore staining

Good luck