There are numerous causes of plant leaves yellowing, most related to nutrition although a few might be related to insect pests. I would start with a good, reliable soil test to see what the soils pH (affects a plants uptake of nutrients) is as well as what the macro nutrient levels are. Keep in mind that too much of one nutrient can keep a plant from using other nutrients, too.
Digging in and taking a good look at the soil these plants are growing in would be a good idea also and these simple soil tests could be of some help,
1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains� too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

jenny.. WHERE ARE YOU ....

as of midnight ... its november..

i defy you to name one plant in the northern hemisphere.. that does not have spotted or yellowing leaves ... ok thats an overstatement.. but you get the idea ....

kim give a rather inclusive list of potential things to think about all winter..

but all you need today.. is a post in the iris/perennial forum.. about how to do a full fall cleanup .... and then see how the garden reacts next spring ....

e.g. ... breaded iris are all showing spotting and ugly leaves.. and because of iris borers.. this time of year.. you trim them all down to about 3 inches.. thereby cutting off all the uglies.. and getting rid of the borer eggs ...

i am not familiar with your iris ... so good luck with that ...

ken

Umm, OP is in Australia.

But yes, we need to see images to know what stage of growth the bulbs are in, then what might be the problem, if any.

Ken, the poster is in Australia and experiencing Spring in her garden.

Jenny, is it possible for you to post a photo - that could help.