Sydney is expected to swelter through another heatwave, with consecutive days of humidity and heat predicted from Tuesday to Saturday.

The "severe" heat is expected to break an eight-year-old record, with five days over 30 degrees in a row not recorded at Sydney observatory since 2011.

Bureau of Meteorology Duty Forecaster Rob Taggart said the Bureau has declared a "severe" heatwave for the Sydney basin, with Western Sydney expected to see "extreme" heatwave levels.

"There is a high-pressure system forming over the Tasman sea, and at the same time there is an inland trough coming from Central Australia," he said. "The system is basically not allowing cold fronts to come through and the trough is directing the hot air mass."