In the past 100 years the average temperature in the harbour city has increased by 1.47 degrees.

"That means we can hold a hell of a lot more water in the atmosphere, meaning we're more likely to see heavy rainfall events," the Bureau of Meteorology's Gabrielle Woodhouse told reporters.

The previous annual record of 2194mm of rainfall, which had stood since 1950, was broken on Thursday.

Since January more than 2157mm had fallen at Sydney's Observatory Hill at 9am, when heavy rain took the total 54.2mm higher by 1.30pm, breaking the previous top mark.

With nearly three months remaining in 2022 and the declaration of a third consecutive La Nina by the BOM in September, more rain is expected to fall before the end of the year, further inflating the record.

For the remainder of spring through summer, the bureau is expecting the rainy conditions to persist.

"We're probably not going to see that break down until we see La Nina start to weaken and move back into neutral territory," Ms Woodhouse said.

The ongoing deluge will see rivers on and west of the Great Dividing Range hit by moderate to major flooding, affecting towns including Tamworth, Dubbo and Bathurst.

There is a severe weather warning for heavy rain and thunderstorms across inland NSW, with no reprieve in sight this week.

Australian Associated Press