North-west Victoria and the NSW Riverina are about to be hit by a "traffic jam" of weather systems, bringing unseasonably-high temperatures for September.
Bureau of Meteorology senior forecaster Dean Narramore said a very-large and slow-moving high-pressure system was taking a week to move from Echuca to the Tasman Sea.
"Because of that, we are in for a prolonged run of warm, and almost hot, sunny weather as we see multiple days of north and northerly winds dragging down that air from central and northern Australia," Mr Narramore said.
"It's almost like a traffic jam of weather systems, so everything gets caught up and slows down, we happen to be under that traffic jam."
Temperatures could reach the low to mid-30s in Mildura, Ouyen and Swan Hill.
The bureau has already predicted a hotter, drier spring.
Mr Narramore said it was also a very-large system, covering much of Australia.
"This heat is looking like it's going to be maintained until at least early next week, with temperatures of up to 8-12 degrees above average, particularly on the weekend and early next week," he said.
The bureau was predicting at least five days of 30-degree or above temperatures for some centres.
"It is unseasonable, particularly when it comes to the prolonged nature of it," he said.
But he said relief was on the way.
"At this stage, it seems we will see a front move through on Tuesday or Wednesday next week," he said.
"That could have some showers and probably lots of wind with it - (but) at this stage, it doesn't look like a big, soaking weather system, more like that hit-and-miss shower activity."
The heat follows a great start to the season, for many crop producers in the north-west.
Manangatang's Brian Barry senior said the heat would test the crops.
He grows wheat, lentils and barley with his son, Brian junior, in the Manangatang area.
"The heat will make the crops work so hard to extract moisture, because of the heat it might be a fair test," he said.
Barley was flowering and filling, while wheat was starting to emerge from the shot blade, the grain stalk that encloses the developing head.
"If we can't get rain, the one thing we would like is cool and mild days - without frost," he said.
"I am asking a lot, aren't I?"
Mr Barry said he was "always cautiously optimistic.
"I use both words with the same weight, you have to be, in this business."
He said cereals could take extreme temperatures - "it's amazing as to how tough our cereals are."
Rupanyup's Andrew Weidemann said the hot spell would hit crops, but the impact would be mitigated if there was follow-up rain or showers to revive them.
"At this time of year it's unseasonable to have those sorts of temperatures, it's not what we are expecting," Mr Weidemann said.
"With these type of temperatures, it's pretty hard for a crop to draw that type of moisture out of the soil, so you will see wilting and that generally means some sort of yield penalty.
"At the back end of October it would be okay, because the crop would be through that maturity phase, enabling it to cope with warmer weather."
He was still hoping for a reasonable season, "barring frost and not too much of this heat."