Dunno, try here.> Pepper Wilt On Plants – What Causes Wilting Peppers

I agree with isgen on this. I'm curious to know what kind of medium they're in. Doesn't look like there's much perlite (if any at all) in those pots. Watering every 2-3 days is quite a lot, but you don't say where you're located. My guess would be that the roots are "choked out" due to soil compaction causing a lack of oxygen from overwatering.

@ isgen/Mike The issue I have with your observation is the size/age of that plant & "I got my chilli plant from a market about 3 weeks ago and had no problems with it until last week.".... Just seems weird to me the plant got that big & OP had no issues for almost a a month then -BAM!- the media is compacted? Not ruling that issue out but just seems weird, no?

Thank you for your prompt replies. I am not too sure about the medium of the soil, but I might try to transplant them and update on the progress. I think you might be right about the over-water and lack of oxygen in the roots because I live in Adelaide, in South Australia, and the weather here can be very harsh and dry. So that's why I thought maybe watering every 2-3 days would be alright.

Not sure what is available down under but look for something with perlite to form air pockets in the media. Do not use garden soil as this really compacts!

Do you think this Potting Mix for Vegetables and Herbs can help my plant and do I have to mix it with perlite or, just that potting mix would be enough?

That's the problem with your Aussie sites - we have no experience with the products available there - even if the brand is available in the US it's not the same. Scott Potting Soil Search ... For many years I've used Miracle-Gro Products and have never been disappointed and other than adding some perlite when re-using + a little compost and fresh peat moss.

In reviewing the description I would believe it has some water crystals added but I haven't a clue how that helps?

This is from a link above. It reads well. If nothing else it is worth a notation, in my mind at least.

Peppers Are Wilting: What To Do For Wilted Pepper Plants - If your peppers are wilting suddenly, developing large yellow areas and drooping (especially if this starts on the bottom leaves and moves upward) despite adequate watering, fungal wilt is probably to blame. Spotted wilt virus is a less common cause of wilted pepper plants, but if your plant’s leaves are dotted with brown or black spots or unusual yellow lines or circles and the symptoms move through the plant from the top down, it is very likely the cause.
Occasionally, bacterial pepper wilt may affect your plants. Pepper plants will wilt and die quickly and upon inspection, the inner stems may be dark, watery and hollow.

What was the soil like when you dug into it before repotting … was it wet and soggy? still looks like you could use some perlite ...