Severe Tropical Cyclone Althea was a powerful tropical cyclone that devastated parts of North Queensland just before Christmas 1971. One of the strongest storms ever to affect the Townsville area, Althea was the fourth system and second severe tropical cyclone of the 1971–72 Australian region cyclone season. After forming near the Solomon Islands on 19 December and heading southwest across the Coral Sea, the storm reached its peak intensity with 10-minute average maximum sustained winds of 130 km/h (80 mph) – Category 3 on the Australian cyclone scale. At 09:00 AEST on Christmas Eve, Althea struck the coast of Queensland near Rollingstone, about 50 km (30 mi) north of Townsville. Although early weather satellites provided only occasional glimpses into the cyclone's formative stages, its landfall was monitored closely by land-based radar that depicted an ongoing eyewall replacement cycle. Althea produced copious rainfall over central and western Queensland as it turned toward the southeast, and on 26 December the cyclone emerged over open waters. After briefly re-intensifying, the system dissipated on 29 December.

While moving ashore, Althea generated wind gusts as high as 215 km/h (134 mph) that wrought significant destruction around Townsville and left nearby Magnetic Island in a state of ruin; almost all of the buildings on the island were damaged to some degree. A significant 3.66 m (12.0 ft) storm surge battered the mainland, while high waves destroyed roads and seawalls. Beaches receded by up to 15.8 m (52 ft) due to coastal erosion. In Townsville, thousands of homes were damaged and many were destroyed. The most widespread damage was to roofs, which were often poorly or not at all secured. The damage from Althea prompted Queensland to develop its first statewide building codes, requiring new homes to be cyclone-resistant. Townsville was the first community to adopt the enhanced construction standards. In addition to the coastal effects, Althea triggered extensive river flooding in interior Queensland. Most major roads in western portions of the state were cut off by floodwaters, and hundreds of families had to leave their homes for higher ground. In the aftermath of the cyclone, 600 Australian Army soldiers aided recovery efforts in Townsville and Magnetic Island, while both state and federal governments contributed disaster relief funds. Three people were killed and damage totaled $120 million (1971 AUD).

Cyclone Althea originated in an expanding area of thunderstorm activity near the Solomon Islands in mid-December 1971. Although little is known about the storm's genesis because of sparse reports and infrequent weather satellite images, the disturbance is thought to have organised into a tropical low on 19 December as it tracked slowly southwestward into the Coral Sea. According to the Bureau of Meteorology's (BoM) Tropical Cyclone Warning Centre (TCWC) in Brisbane, the system reached tropical cyclone status around 06:00 UTC on 20 December, less than 300 km (200 mi) southwest of Honiara. Just after 23:00 UTC on 21 December, imagery from the United States ESSA-8 satellite showed the first indications of an emerging eye feature. Althea tracked southwest toward the coast of Queensland around a subtropical ridge to the south, and steadily strengthened. Near 18:00 UTC on 22 December, it reached its peak intensity with 10-minute maximum sustained winds of 130 km/h (80 mph); this made it a Category 3 severe tropical cyclone on the Australian scale. The cyclone's central barometric pressure of 952 hPa (28.1 inHg) was estimated using the lowest recorded air pressure, 971.5 hPa (28.69 inHg) at Townsville Airport, a 32 km (20 mi) radius of maximum wind, and environmental air pressures.

Late on 22 December, the eye remained obscured to satellite imagery by a canopy of cirrus clouds while the cyclone passed north of Lihou Reef. Early the next day, it tracked just south of the automated weather station on Flinders Reef. Althea likely continued to organise until landfall, which occurred at 23:00 UTC on 23 December – 09:00 AEST on Christmas Eve – near Rollingstone, about 50 km (30 mi) north of Townsville. The landfall point placed Townsville and nearby Magnetic Island in the cyclone's powerful left-front quadrant. Because of the tight pressure gradient between Althea and the high pressure area to the south, gale-force winds extended well to the south of the cyclone's centre. While the system was moving ashore, land-based radar imagery identified an ongoing eyewall replacement cycle, with two distinct, concentric eyewalls – rings of intense thunderstorms surrounding the centre. The strongest winds were found under the contracting outer ring, which shrank from 55 to 39 km (34 to 24 mi) between 21:00 and 23:00 UTC to become the dominant eyewall as the inner ring dissipated. Consequently, extreme winds initially extended relatively far from the centre.

The storm quickly weakened as it continued southwest, passing just north of Charters Towers City, though it continued to produce heavy rainfall over interior Queensland. Curving southeastward, Althea re-emerged over open waters between Maryborough and Double Island Point on 26 December and began to intensify once again. At 00:00 UTC on 28 December, the cyclone reached a third peak with 10-minute winds of 110 km/h (70 mph), but as it turned more toward the south, increasingly cooler sea surface temperatures weakened the cyclone. On 29 December, Althea lost its tropical characteristics over the Tasman Sea.

In the hours leading up to landfall, the TCWC in Brisbane issued 17 cyclone warnings for coastal areas. According to the BoM, residents received 20 hours of advance warning in which to complete preparations. By late on 23 December, police and state emergency service workers were on alert for the approaching storm. High public preparedness and a relatively low loss of life were attributed to BoM cyclone awareness initiatives in the wake of 1970's deadly Cyclone Ada. However, storm victims still criticised the agency and local radio news stations for unclear, untimely, or conflicting broadcasts that caused widespread confusion. Significant delays between the issuance of warnings from TCWC Brisbane and local broadcast in the Townsville area raised some question about the need for a warning centre closer to North Queensland.

Total damage from Cyclone Althea amounted to just under A$120 million (1971), and the normalised damage total for 2012, which accounts for growth and inflation, was estimated at $648 million. Throughout the affected region, 257 people were treated for storm-related injuries, mostly inflicted by airborne debris; three people were killed. In Townsville, ten people were reported missing during the cyclone: nine on three boats that were unaccounted for, and one whose car was found in a swollen creek. They were all found safe.

An estimated 90% of the buildings on Magnetic Island were damaged to some degree by 215 km/h (134 mph) wind gusts, and there was "not a tree left with leaves on it". The island's main town of Picnic Bay was hit particularly hard. About 100 people endured the cyclone in the dining room of a Picnic Bay hotel that ultimately sustained severe structural damage; 65 would remain sheltered there through Christmas Day. Around 60% of houses in Nelly Bay and 40% in Arcadia were demolished. In the village of Horseshoe Bay, one woman died in a building collapse, and reportedly only 8 out of 150 houses in the community survived the cyclone. The island suffered a nearly total power outage, and after the water pipeline from the mainland was damaged, strict usage rations were put in place until repairs could be made. About 30 people on Magnetic Island were treated for injuries sustained during the storm, and damage reached $2 million.