'It's devastating': Woman's heartbreak as sister goes missing in Townsville floods

A woman is desperately trying to reach her sister and her two young children as floodwaters rise, stranding some people inside their houses in Townsville, Queensland.

It's been six days of monsoonal rain for the city, warnings given to over 20,000 homeowners that a quarter of the city's houses were at risk of severe flooding.

People are being warned to stay out of the water due to strong currents, snakes and crocodiles making it too dangerous to enter.

Milly Roberts told 7 News she hadn't seen or heard from her sister or her sister's young children since power went out on Sunday, and she fears for their safety.

"Our whole family is worried and we just want her at my house where she will be safe and not in all of this."

"It really is heartbreaking and like I said, we haven’t heard from her and we all just want to know she's okay."

She said she had planned to drive as far up to the water as she could, then walk along the fence line to her sister's house, but with floodwater still rising, this proved impossible.

"There's no possible way I could do that, I tried everything in my power to get them to me and unfortunately none of that worked."

She said the high water level meant her sister had most likely lost everything on the ground floor of her house.

"She has probably lost everything except for the stuff upstairs. It is really devastating not just for our family, but everyone's families."

Authorities said the water is set to continue rising as flow through the open dam slows.

The dam had to be opened, letting water rush through the city, after concerns emerged about its strength when it hit 100 percent capacity.

Almost 2000 cubic metres of water is rushing out of the Ross River dam, towards Townsville every second.

Rescue crews are working tirelessly to rescue residents who could not evacuate in time.

Newshub.