BoM duty forecaster Gabrielle Woodhouse said up to 15 millimetres could fall on Sunday and another 20 millimetres and thunderstorms were expected on Monday.

"That trough sitting over South Australia is going to slowly move east and that's what's going to feed this quite warm and humid air in front of it," she said.

"That's going to lead to the showers and thunderstorms as the trough itself moves further towards eastern NSW.

"That's why we'll see showers and storms developing over inland NSW tomorrow and then shifting across to the ACT and the eastern parts of NSW come Sunday."

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Much of the rain is forecast to soak the Brindabella Range with Canberra still expected to get some of the action.

Temperatures will remain relatively spring-like with a top of 21 on Saturday, 19 on Sunday and balmy 23 on Monday.

Ms Woodhouse said there remained a 70 per cent chance of La Nina forming this spring, which would bring widespread wet weather.

"What we're seeing at the moment is a really humid air mass feeding in ahead of the trough so you've got the moisture there to be able to create the clouds and the rain," she said.

"It's on par with our expectations if we're leaning more towards the La Nina forming, it's more likely we see these sorts of systems because there is the moisture there to be rained out."

Ms Woodhouse said parts of Western NSW which had missed out on a drop during recent rainfall over the east of the state, would experience some wet weather this weekend.