For Kuala Lumpur you should dress however you are comfortable (okay, not naked, you need to have some level of modesty); you'll be okay walking around the city in a sleeveless t-shirt, shorts and flip-flops/thongs. You probably won't be dressed appropriately for some venues, but you're fine for standard tourist activities. This dress is a bit uncommon for males, but trust your sense for what's appropriate.
However, if you could purchase a collared shirt made with wicking material (Underarmor or equivalent), some nice wicking shorts that require a belt (like for golf) and trainers you'll be good just about anywhere.
KL is an international city, with a population of 1.6 million which places is between Perth and Adelaide compared to Australian cities1 2. With three cultures resident in Malaysia (Malay, Chinese, and Indian) there is a lot of diversity.
The Malays (generally Muslim) tend to dress more conservatively, but there are some pretty liberal ethnic Malays in KL. My experience is the Muslims dress conservatively for socio-family and personal reasons. They don't try to force their values on you.
The Chinese tend toward very Western and the women are not afraid to show cleavage, wear form fitting clothes, dye their hair, etc. I only mention this is a gauge of how conservative things are/aren't.
The Indians, the overwhelming minority, span the width from sari's to conservative Christian or Muslim to Western.