Police are hunting for a gun-wielding, balaclava-wearing woman who held up one of the country's largest military bases on Friday afternoon.

The Royal Australian Air Force base at Richmond, on the north-western outskirts of Sydney, was placed in lockdown on Friday evening after the woman robbed a bank within the heavily fortified base.

Richmond is the hub of logistics support for the Australian Defence Force and houses several of the force's biggest units, including C-130 Hercules aircraft and an intelligence squadron.

It has its own post office, military credit union and public transport network.

The woman, dressed in all black and wearing a balaclava, entered the financial institution with a gun just after 3.30pm and threatened another woman working there.