The pair are thought to have been riding in the back of a utility swept off the road while being driven across a flooded causeway near the South West Slopes town of Boorowa on Monday night.
Police say two others in the cab were able to free themselves and get to safety.
The search was briefly paused in the early hours of Tuesday morning but was ongoing in the afternoon.
Further west where Muttama Creek divides the Riverina centre of Cootamundra, residents have been directed to emergency accommodation at the local showground, with dangerous flash flooding a concern.
Local member and NSW Emergency Services Minister Steph Cooke told the ABC the order was for 10 streets and about 100 homes.
"The weather event we saw come through from late yesterday afternoon and overnight has really caused some substantial damage and wreaked havoc right across the region," she said.
"It's a long time since we've seen flooding hit the town in the way that it did last night and so people were having to work closely with the SES."
The Murrumbidgee River at nearby Gundagai exceeded major flood levels (8.5m) on Tuesday afternoon, rising more than 2.5m in 24 hours.
The Bureau of Meteorology warns it could be the area's worst flood since 1989.
Evacuations have been ordered and a series of road and bridge closures are also likely.
Downstream at Adelong, the main street is under water and residents in adjoining streets have been ordered to leave.
An evacuation order has also been issued for Tumut Caravan Park on the Tumut River amid rapidly rising waters and the threat of flash flooding.
Cowra Caravan Park on the Lachlan River is also under evacuation orders.
Major flooding is again likely along the Castlereagh, Gwydir, Namoi and Macquarie rivers after recent rain.
The BOM also expects major flooding on the Bogan River.
There are 90 SES hazard warnings current for NSW.
They include evacuation orders for residents along the Murray River at Willow Bend, Moama and Mathoura.
After some sunshine to round off October, the state faces another rainy month as temperatures drop, the BOM says.
Showers and thunderstorms were forecast across most areas on Tuesday, with creeks and rivers likely to rise rapidly as rain falls on already saturated or flooded catchments.
The cold front sweeping Australia's southeast will bring chilly temperatures, showers and alpine snow until midweek - all of which are unseasonal for November.
Above-median rainfall is forecast for most of eastern Australia over the next two weeks, increasing to very likely (more than 80 per cent chance) for the northeast and far southeastern mainland.
16°C