They will arrive on the Australian mainland within hours of each other, but cyclones Lam and Marcia will have very different impacts, an expert says.
Severe tropical cyclone Lam should cross the Arnhem Land coast near Elcho Island late on Thursday or early Friday.
The small, intense and slow-moving category three storm has 140km/h winds near its centre and gusts to 195km/h. It's 95km northeast of Elcho Island.
Earlier predictions suggested it would intensify to category four before crossing the coast, but is now expected to maintain category three strength.
On Queensland's Capricornia coast, Cyclone Marcia's winds will not pack the same punch as Lam's, but will cause major flooding from Mackay to northern NSW, with hundreds of millimetres of rain expected in coming days.
Category two Marcia has sustained central winds of 95km/h and gusts to 130km/h but is now tipped to intensify to severe category three by early Friday before landfall.
The Bureau of Meteorology's Alan Sharp says Lam has similarities to Cyclone Tracy.
"Being a small system, it has had a chance to intensify. Cyclone Tracy was very small and intensified quite quickly when it moved into Darwin back in 1974 and wreaked its havoc," the national manager of tropical cyclone warning services told AAP.
"The sea surface temperatures up there are very high, so it's had ideal conditions."
Lam's arrival heralds a late cyclone season in Australia.
Mr Sharp said it was only the third cyclone in Australian waters since the season began on November 1, and would be the first to cross the mainland coast.
Marcia is likely to cause dangerous storm tides and the worst flooding in Queensland since Cyclone Oswald in February 2013.
"(Marcia) is further south and the conditions aren't favourable for its formation. It will produce a lot of rain once it gets over the coast," Mr Sharp said.
"(Oswald) hardly got to category one but as it moved down the Queensland coast it caused major flooding."
Lam would deliver widespread flooding in Arnhem Land, Mr Sharp said.
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