Forecasters say heavy rain may lash southeast Queensland and northern NSW this week, with Seth churning about 700 kilometres to the northeast of Brisbane on Sunday afternoon.

"There is quite a large stretch of easterly winds to the south of the system and that's driving quite a bit of swell and wave action onto the Queensland coast as well as parts of the Northern river coast of New South Wales," Bureau of Meteorology senior meteorologist Jackson Browne said.

"We've seen water levels over 30 centimetres above what would normally be possible with the tide and we have also seen some minor nuisance flooding."

The category one system is expected to weaken as it moves toward the coast, but will remain capable of creating hazardous conditions.

"We should see a pickup in the wind which is possible around some of those exposed locations," Mr Browne said.

"We do have a severe weather warning out for abnormally high tides stretching from the Byron and Coffs coast in New South Wales up to the Fraser Coast in Queensland."

Remnants of Seth are expected to cross the coast on Thursday or Friday, bringing potentially heavy rainfall.

"The guidance is very splintered as to where it may possibly go - anywhere from Bundaberg down to around the Gold Coast.

"That will dictate where the heaviest rain will fall."

Queensland Police Assistant Commissioner Shane Chelepy says like all cyclones, Seth is unpredictable and being monitored closely.

"What we are expecting to see is very high tides on the coast in our low-lying areas, potentially some flooding as a result of those high tides, and some severe weather action on the coast with respect to waves and damaging surf," he told reporters in Brisbane on Sunday.

"It is really important that anybody going to the coast today and over the next couple of days is very mindful that they will be significant surf on the coast, damaging waves, and some very high tides which will cause rips along the coast and some local flooding."

Seth was downgraded from category two to a category one on Saturday.

BOM has issued a severe weather warning from Wide Bay to the southeast coast with high tides expected to reach an "astronomical peak" over the next few days, easing on Wednesday.

Areas that may be affected include Gold Coast, Maroochydore, Coolangatta, Moreton Island, Noosa Heads, Caloundra, North Stradbroke Island, Rainbow Beach and Redcliffe.

Australian Associated Press