A firefighters' union has slammed NSW Premier Gladys Berejiklian for denying there have been budget cuts to the state's fire services.

Fire Brigades Employees Union NSW secretary Leighton Drury said Fire and Rescue NSW was currently 400 people short of establishment numbers.

"Since 2011, when this government came in, they imposed a labour expense cap on non-frontline workers (and) they classify firefighters as non-frontline," Mr Drury told reporters in Sydney on Thursday.

"Since then, every year up until now, Fire and Rescue NSW has been imposed with, up to this year, $20 million for this year alone in budget cuts.

"We're 400 firefighters short by our count.

"That means boots on the ground that aren't there."

FRNSW is the state's urban firefighting body, while bushfires are typically handled by the Rural Fire Service NSW.

Ms Berejiklian and her government has repeatedly denied union claims they had cut almost $13 million from the FRNSW budget.

She also denied the state's firefighting bodies were understaffed.

"That is completely incorrect and wrong. In fact whenever I'm updated at RFS, the fireys - all the agencies - are working together more closely than I've ever seen," Ms Berejiklian told ABC Radio on Tuesday.

The RFS has confirmed more than 300 homes have been destroyed or damaged since Friday, at least 50 of them on Tuesday alone.