Winter might be nine days away but Sydney is forecast to swelter through a week of temperatures as high as 27C – eight degrees above the average maximum for May.

At the same time, Melbourne is facing eight consecutive days above 20C – an unseasonably hot run that has only happened five times before in the month of May.

With autumn nearly over, the Bureau of Meteorology confirmed on Tuesday that this year’s autumn was “significantly warmer” than the long-term average, by 1.5C.

Combined with the hottest summer, and the hottest March on record, this means 2019 has been Australia’s hottest start to a year since records began.

On Tuesday Sydney was forecast to hit 26C in the city centre, and 27C in the west. The average maximum temperature for May is 19.5C.

Ashleigh Lange from the Bureau of Meteorology said this was due to a high pressure system “being the dominant feature of the past couple of days”.

“For the next week, we’ve got above-average temperatures, and it’s not until Monday when we see a drop down to 21C,” she said.

Sydney is forecast to reach 25C on Wednesday, 23C on Thursday, 25C on Friday and Saturday, and 26C on Sunday.

The highest temperature recorded in Sydney in May was 30C, on 1 May 1919.

In Victoria, maximums will be at 20C or higher for the rest of the week – 4C higher than the average maximum for May of 16.7C. Melbourne is forecast to have eight days above 20C.

According to the bureau, April was also hotter than average – and among the top 10 hottest recorded for New South Wales, South Australia and the Northern Territory. Parts of NSW and Western Australia experienced their hottest April days on record.

The combination means it has been the hottest January-to-May period in Australia’s recorded weather history – and one of the driest.

Blair Trewin, a senior climatologist at the bureau, said nearly the whole country had experienced a hotter autumn that average.

“It’s clear that this autumn is going to come in significantly warmer than average. It’s not going to be a record warm autumn but it is certainly going to be significantly warmer.

“Australia is continuing to be on track for its warmest start to the year on record. It’s also been the driest start to a year on record for Melbourne, and significantly drier than average for Hobart, Adelaide and Perth.”

The bureau’s forecast for the coming winter also predicts it will be warmer than average, and drier.

Winter is a time when farmers “are relying on fairly reasonable rain”, but there is a “70% to 80% chance of below average rainfall in northern Victoria and southern NSW,” Trewin said.

All of the bureau’s season forecasts this year say natural events, including El Niño and the Indian Ocean Dipole, and the long-term trend of human-caused global heating, have been the cause of the record-breaking hot weather.

“The forecast for the next couple of months is for warmer-than-average winter days and winter nights,” Lange said.

On Tuesday the bureau also issued an air pollution warning for Sydney, owing to smoke from hazard reduction burns.