A woman has been arrested after allegedly vandalising water and gas mains at two separate hospitals in Sydney's south this morning.

Police responded to reports of vandalism overnight at Sutherland Hospital and Kareena Private Hospital on Kareena Road in Caringbah.

Staff at the first hospital told police a 42-year-old woman had entered the grounds and cut its water and gas mains, before activating fire hoses, about 12.20am today.

It is alleged the same woman entered a second hospital on Kareena Road at 1.30am and switched off the gas main.

The woman was arrested on Parkside Avenue in Miranda for breach of bail shortly before 4am.

She has been taken to Sutherland Police Station and is speaking to police.

An investigation into the alleged vandalism is ongoing.

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