score:3
240V will work fine. Mains voltage and anything that uses it will have tolerances of at least 6% (~14V for 240V). Making use of this, the Australian voltage change was mostly done on paper, since most actual Aussie electricity is still served at 240V.
That said, please ensure that any devices you plug into your transformer are rated to use less than 1500W peak. The breast pump will be fine, but heavy-duty blenders and mixers may push the limit, and overloaded transformers can catch fire and/or electrocute you. Don't plug a power strip into your transformer, don't leave the devices running for long periods and keep the transformer turned off while not in use.