Fire and Rescue NSW has defended the fact it had fire stations in Sydney that were closed as bushfires burned out of control on the edge of the city.

A house burned down and two firefighters were injured in fires across western Sydney and in the Blue Mountains on Tuesday with about 1,000 firefighters battling the blaze.

The Fire Brigade Employees' Union (FBEU) attacked the New South Wales state government on Tuesday night, claiming five fire stations across the city were closed and four were vacant as their firefighters were sent to cover positions in other stations.

"You don't have to be a rocket scientist to know that hot weather and strong winds can mean bushfires, but it appears to be news to the O'Farrell government who have been caught with their pants down today," the FBEU secretary, Jim Casey, said.

"Off-duty fire crews had to be called in as neighbouring stations raced to protect life and property, all because budget cuts have left large areas of Sydney without adequate fire protection.

"Budget cuts and the irresponsible policy of closing fire stations meant that as fire conditions developed into a major emergency today, fire stations across the city were shut."

About $64m was cut from the NSW Fire and Rescue budget in employee benefits last year which meant a reduction in staff. The organisation was also set a $11.7m savings target over four years by the state government.

The union claimed fire stations in Camden, Riverwood, Miranda, Mona Vale and Ryde were closed because of budget cuts and crews from Macquarie Fields, Ashfield, Botany and Newtown covered the areas, leaving their own fire stations vacant.

Fire and Rescue NSW rejected the union's criticism but admitted to leaving fire stations vacant as firefighters were sent to other stations.

"Due to staff absences, yesterday fire stations at Mona Vale, Riverwood and Miranda were backfilled with firefighters from other stations – they were not closed or unattended," a spokeswoman said.

"Ryde, Botany, Ashfield, Macquarie Fields and Newtown were unattended because they are in areas where a number of other nearby fire stations can respond within FRNSW time standards.

"There was a shortage of on-call retained firefighters at Camden fire station. Full-time firefighters from Macquarie Fields were assigned to respond from that station. It was not closed or offline."

Fire and Rescue NSW cancelled all training at 12.30pm on Tuesday when it realised weather conditions were worse than forecast.

The training cancellation meant 10 trucks were put back on the road immediately and off-duty firefighters were recalled putting an extra four trucks on the road. By 2.30pm all permanent fire stations were online, according to the FRNSW spokeswoman.

"The FBEU's claim that resources would have been stretched if there had been a factory fire or house fire cannot be substantiated," she said,

"Fire & Rescue NSW maintained capacity in the city and suburban areas to respond to other emergencies while deploying significant resources to the bushfire emergency.

"This demonstrates Fire & Rescue NSW's ability to rapidly and flexibly deploy firefighting resources where and when they are needed. Each fire truck and its crew is a mobile resource, part of a large network of crews and fire trucks available to respond wherever they are needed, as demonstrated yesterday."

Investigation into the four bushfires in Sydney's west and in the Blue Mountains are under way and the Rural Fire Service has not ruled out that they were deliberately lit or caused by hazard reduction fire which got out of control in dry conditions with high winds.

The RFS are doing about 100 hazard reduction burns in the lead up to summer but have not said if there were specifically any in the areas of the bushfires over the past week.

The NSW premier, Barry O'Farrell, defended the burns, saying the alternative was "disaster".

"We are deliberately encouraging as much hazard reduction as possible," he told reporters in on Wednesday in Winmalee, about half a kilometre from one of Tuesday's biggest blazes.

"When you don't undertake hazard reduction you leave the fuel load as it is and it grows and that's even more lethal."

O'Farrell also paid tribute to firefighters – paid and volunteer – who were battling bushfires in Sydney's west.

"It's clearly going to be a difficult bushfire season," he said.