A 22-year-old Sydney woman is feared dead after being swept away in a swollen stream while tramping in New Zealand.

Searchers will on Wednesday assess weather conditions before deciding whether to resume their hunt for a missing woman, who was overwhelmed by the water on the Milford Track in the southwest of the South Island late on Monday.

The woman, described by one official as Indonesian, was walking the track with her Australian partner.

German backpacker Sebastian Keiholz, who was walking with the pair, told Fairfax he knew them as Jessica and Jean, and they were from Sydney.

The two men managed to cross the stream but Jessica got caught on a rock in the middle and panicked and fell. She was swept away.

Mr Keiholz activated an emergency locator beacon and ran to the nearest hut.

He still had to make another dangerous stream crossing and said he wished he was warned about the dangerous crossings.

“I’m angry that they didn’t tell us that there are no bridges and I’m angry that I couldn’t help her.”

Poor weather meant a helicopter could not search immediately for the missing woman on Monday.

Air searches were conducted morning and afternoon on Tuesday, but the weather has prevented a foot search of the area.

Police say the weather will dictate whether a search is carried out on Wednesday.

They have not yet confirmed the name and nationality of the missing woman.