Flood victims will not be able to return to their homes until their safety can be assured, a council leader has said.
The Coal Authority has said initial checks suggested water built up in a mine shaft causing a "blow out" that flooded properties in Skewen, Neath Port Talbot.
About 80 people were evacuated as water rushed through the village on Thursday.
Council leader Rob Jones said it was unlikely residents could return Monday.
He said underground investigations would begin on Saturday and the work could take two to three days.
"Safety is the paramount concern for us," he said.
"Because we can't guarantee the site safety - that's the reason why people will remain away from their properties until such time as we can give the all clear.
"We don't know what the water has done underground."
The fire service said on Saturday morning the pumping operation was "making good progress".
Mr Jones told BBC Radio Wales Breakfast people may be able to return next week but "did not want to raise hopes" it will be Monday.
He said the flooding was "more than likely" related to old mine workings with six mines known about in area. He said the industry dated back 300 years.
Skewen resident John Thomas returned home from a funeral with wife Lynne on Thursday to find their house had turned into "a lake".
He said: "The water was around the level of the bottom of the doors so we couldn't go in, so we just had to stand there and watch this orange-coloured water just piling up and up and up.
"Other people who were evacuated had the chance to move things upstairs, I didn't have a chance to do that because I couldn't get in to it."
Local MP Stephen Kinnock said affected residents were staying in "lots of different places" across the region.
And he praised the "extraordinary" generosity of the community and the support of the Salvation Army with donations of food, clothing and toiletries.
⚠️SKEWEN FLOOD UPDATE⚠️— Stephen Kinnock (@SKinnock) January 22, 2021
Earlier today I was in #Skewen to see the devastating impact of yesterday's flooding.
My office & I are here to help you if you have been affected by the flood. If you require our assistance please email me at email@example.com. pic.twitter.com/fQ448y1Y5e
Natural Resources Wales (NRW) said officers were continuing to look at how to minimise the risk of pollution to nearby rivers, and investigating any impacts on the River Neath.
The Coal Authority, which manages the effects of past coal mining, is investigating the incident.
Chief executive Lisa Pinney said equipment, due on site on Saturday, would be used to drill into mine workings to "fully investigate what has happened".
"The blow out is likely to have been caused by a blockage underground which has caused water to back up and to break out using the easiest path," she said.
"The excessive rainfall of the past few days and the prolonged rainfall this winter, will have put additional pressure on the system.
"We know that people will want to get back to their homes and we will continue to progress these works as soon as possible, but public safety has to come first."
There are a number of historical mine workings in Skewen dating back beyond 1850.
On Saturday, Mr Jones said water was still pouring out of the affected site so workers were diverting it, while machines cleared gulleys and drains to give the water the chance to enter drainage systems.
A residents' incident support centre has been set up at Abbey Primary School to offer help and information over the weekend, between 09:00-17:00 GMT.
The council has asked residents to be "patient as the investigation continues" and has set up a helpline. Tel. 01639 686868.