The first big heatwave of the summer will scorch inland Australia this week and reach the east coast by Thursday, sending the mercury and fire risks soaring as it goes.

According to the Bureau of Meteorology, about half of NSW will begin experiencing a low-intensity heatwave from Monday that will strengthen.

By Wednesday, more than a third of the state will have severe heatwave conditions, with a similar day forecast for Thursday.

Sydney's maximums should be moderated by coastal sea breezes most days this week, although Wednesday's top is predicted to reach 30 degrees on Wednesday and 33 degrees on Thursday.