Searchers looking for a tramper missing in rugged country in the South Island have found tracks leading to a river.

The search resumed this morning for the man who has been missing since Sunday evening in Mt Aspiring National Park.

Wanaka Search and Rescue spokesman Phil Melchior said the tramper, in his early 40s, was neither a complete novice nor an experienced bushman.

"We're absolutely confident that we have tracked the man...we tracked him into the river, out the other side, back across. And then we get to a point where we get him going into the river again but unfortunately we've not been able to find any trace of him coming out," Mr Melchior said today.

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He said said sending volunteers to search in wilder parts of the area would be like dumping people in a giant, unstoppable "washing machine."

Some parts of the country were almost inaccessible.

The missing man was last seen about 6.30pm on Sunday after parting company with a group hiking and camping near the 'Jumboland' area beside the upper Wilkin River.

"What the party was doing was perfectly within their capacity. It was a perfectly reasonable thing for a guy like that to be doing until he unfortunately decided to leave the track...".

An Aspiring Helicopters chopper was also involved in the search, which resumed today about 8am, Mr Melchior said.

"They will basically do a fairly detailed aerial search to see if anything's come to light overnight and look again at some of the areas that were looked at yesterday."

Wanaka Search and Rescue had a team of about six volunteers searching today, scaled back from 12 or 13 yesterday.

The team would probably reassess the scale and focus of the search this afternoon.

Yesterday, Wanaka police Search and Rescue co-ordinator Sergeant Aaron Nicholson said a river rescue team and bush search team had looked for the man through the day, while an aerial search also took place.