The Tasmanian Fire Service has extra crews on standby as temperatures around the state climb.

The TFS says hot spots were detected overnight within containment lines of the Risdon Vale blaze near Hobart.

The fire has burnt 420 hectares since Wednesday.

Spokesman Paul Symington says a predicted 33 degrees and 30 kilometre winds could increase the fire's danger.

He says extra crews are on standby with a very high fire danger rating in place in the south and the midlands.

"We are preparing for a warm afternoon and we're just very mindful that when fires do start today they will be quite difficult to control," he said.