Australia-Hottest summer in 100 years farmers worst nightmare.

Victoria and South Australia on Wednesday sweltered in a heatwave which is developing into one of the worst in 100 years.

Adelaide by midday had already exceeded its forecast 44deg.

It hit a peak of 45.7deg during the early afternoon.

SA country centres, all the way from the Mid-North across the farming areas to the South-East corner, copped equivalent heatwave temperatures.

All indications are for SA to be just as hot until the weekend, when a weak cooler change arrives, according to weatherzone.com.au.

If Adelaide also reaches 40 degrees on Thursday and Friday, it will equal the longest such hot spell in 101 years.