Some people in Co Down have been unable to leave their homes after a deluge.
A yellow weather warning for rain has been extended.
Major flooding in Newcastle , Tullybrannigan Road
Watch – Major flooding in Newcastle , Tullybrannigan Road
The alert came into effect at midnight and was due to be in place until midday, although has now been extended to 3pm. It is affecting counties Armagh, Down and southern parts of Co Antrim.
There is a lot of surface water and some flooding on the roads, but none have so far been reported as impassable.
In Co Down, people living in Newcastle have been using sandbags to protect their homes after flooding.
Alliance Party councillor Jill Truesdale and described seeing the water “funnelling down the mountain”.
“I’ve never seen so much debris come down before, it’s quite frightening,” she told BBC News NI.
She and her family are now unable to leave their house due to the flooding.
“It looks as if we are going to be [trapped] for a while because there’s also a high tide so we won’t be leaving, it’s not worth the risk,” she added.
The Met Office had forecasted that heavy rain will spread across the whole of the region before gradually clearing to the east on Sunday morning.
Downpours were due to be heaviest in the southeast, with 20-30 mm expected to accumulate widely.
Over the Mournes, 60-100mm were expected to build up much of this falling in the early hours of Sunday morning.
The Met Office added the weather warning will cause some impacts with bus and train services probably affected with journey times taking longer as well as spray and flooding on roads.
Flooding of a few homes and businesses was considered likely along with some interruption to power supplies.
The Met Office advised to the public to check if your property could be at risk of flooding and if so, the Met Office has advised preparing a flood plan and emergency flood kit.
Travellers should avoid delays by checking road conditions and bus and train timetables.