Brett Summerell, director of science and conservation at the Royal Botanic Gardens, said the combination of heat and the lack of rain was affecting plants in different ways at its three gardens.
"It's weird," Dr Summerell said, noting some trees, such as maples, in the Gardens' Mount Tomah site had "been losing leaves, and then resprouting".
For the city's Botanic Gardens, the plants were mostly sub-tropical or tropical species that could cope better with the warmth but had to be well-watered to keep in good condition.
"We're really desperate for some rain," he said.
The Mount Annan gardens were also being hard hit by the unseasonable weather.
"It's horrendously dry out there," Dr Summerell said. "It's a struggle to keep [some of the trees] alive."
The Bureau of Meteorology said the heat across much of the country had been "more characteristic of mid-summer than mid-autumn [and] was unprecedented in many areas in April for its intensity, its persistence or both".
"The spatial extent of the heat was also exceptional, with above-average maximum temperatures extending almost nationwide on each day during the first 10 days of the month," the bureau said in a special climate statement released on Friday.
The heat built up over the north of Western Australia in late March, with some sites recording the first days above 45 degrees after the autumn equinox ever recorded in Australia. It then spread south and east.
April records fell in South Australia, with 42.2 degrees at Nullarbor on April 9. The following day, Mildura set Victoria's April record with 39.3 degrees, and Pooncarrie set a new high mark for NSW with 40.5 degrees – beating a mark set in 1922.
April 9 also set an Australia-averaged record for the month, reaching 34.97 degres, or 0.65 degrees above the previous record reached in 2005, the bureau said.
So far, the event has set hottest or equal-hottest marks at 64 of the bureau's monitoring locations with at least 40 years of data.
While the daytime heat has been extreme for this time of year, some regions have also had unusual overnight readings.
Point Perpendicular, near Jervis Bay on the NSW South Coast, posted its fourth warmest night for any time of the year.
On Monday, Sydney notched its hottest April day on record at 35.4 degrees, beating the previous high by 1.2 degrees.
It was also the latest date in the season anywhere in the Sydney metropolitan region to have surpassed 35 degrees – at least until Thursday produced a batch more.
Sydney's nine consecutive days of 25 degrees or warmer to start April were also a record for the city. More records may be set on Friday, depending on how high the mercury
There's little immediate prospect of a return to more typical April conditions for Sydney until next Wednesday, when a top of 23 degrees is forecast.
Before then, Saturday could make it Sydney's fifth day this month of 30 degrees or warmer weather, on current forecasts. That compares with just 0.4 such days on average for April.
A front should move through over the weekend but it is unlikely to bring below-average temperatures.
"The air mass in the wake of this system is still warm," Weatherzone's Mr Cronje said.
No below average day in April?
Looking further out, there is also little prospect for a significant cold spell before the end of April, he said.
"It's safe to say it could be the hottest April on record [for Sydney], and we could see the daytime maximums not going below the long-run average of 22.5 degrees on any day this month," he said.
Little more than the odd shower is expected for the coming week.
So far this month, the city has recorded no rain but had 78 millimetres of evaporation up to Thursday, the bureau data show. Average rainfall for Sydney in April is 128.5 millimetres.
Peter Hannam is Environment Editor at The Sydney Morning Herald. He covers broad environmental issues ranging from climate change to renewable energy for Fairfax Media.
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