Thickness, When purchasing a wetsuit people often refer to the catalogue thickness of the wetsuit. However a better method of choosing a wetsuit is by referring to your dive log. Australia is 20 degrees of latitude wide across the coastal areas and therefore suitable wetsuits for the far north in October would not be suitable for the far south in August.
Why water temperature is only the starting point
Surface water temperature for locations worldwide provides an approximate indication of water temperature for diving locations around the world. Surface water temperature does not provide an accurate indication of water temperature at depth and often cooler water will exist at depth than at the surface. The coldest water in most locations exists below 15m and can extend into warmer surface water. In southern locations cold upwellings can occur within days and provide little warning to the diver via forecasters.
Duration and exertion change the calculation
A forty minute snorkel around a rock is a completely different thermal event to a four hour spearfishing session, and a four hour active dive is a completely different thermal event to a long float around watching for fish. For the free diver the wetsuit will compress and then expand with each change of depth, thus the free diver will lose more heat than the scuba diver at equivalent depth.
Surface reading of the water temperature will give you a rough guide to the water temperature. However, when you actually go for a dive you will realise that it actually feels colder than what you read on the surface. In southern areas, upwellings can occur within days, and often there is little warning in the forecast.
Comparing 1.5mm, 3mm and 5mm
The thicknesses of wetsuits listed above have real working ranges between them.
| Thickness | Working water temperature | Typical use | Main trade-off |
| 1.5mm | 26°C and above | Tropical reef, summer surface work, sun and sting protection | Almost no insulation reserve if conditions turn |
| 3mm | 21°C to 26°C | Year-round Queensland, NSW spring through autumn, mobile diving | Marginal on long sessions below about 22°C |
| 5mm | 15°C to 21°C | NSW winter, Victorian and South Australian summer, deep east coast diving | Buoyancy and weighting penalty, reduced flexibility |
| 7mm | Below 16°C | Southern winter, Tasmania, extended cold sessions | Bulk, cost, fatigue over long days |
The buoyancy cost of thicker neoprene
The amount of lead needed to make a wetsuit neutrally buoyant will increase with the thickness of the wetsuit. For example, a 3mm wetsuit would need around 2-3kg of lead to make it neutrally buoyant. The amount of lead needed, as well as the effort needed for the ascent, will affect how you set up your spearfishing rig for neutral buoyancy at working depth. Therefore it is not recommended to wear your summer wetsuit with extra lead for winter and your winter wetsuit without extra lead for summer.
Regional guidance across the Australian coast
- Far north Queensland. Water sits between 24°C and 30°C. A 1.5mm suit or a two piece 2mm covers most of the year, with a 3mm for winter mornings and deeper work on the outer reef.
- Southern Queensland and northern NSW. A 3mm is the genuine year-round option, with many divers adding a 5mm for July and August when temperatures fall towards 19°C.
- Sydney and the south coast of NSW. Summer sits comfortably in 3mm territory. Winter drops to 16°C or lower and a 5mm becomes the minimum, with a hooded top for long sessions.
- Victoria and South Australia. A 5mm is the summer suit here, not the winter one. Bass Strait and the western Victorian coast justify 7mm from May through September.
- Tasmania and the Great Australian Bight. 7mm with hood, gloves and boots is standard, and open cell neoprene makes a substantial difference to comfort over a full day.
Building a two suit wardrobe
A 3/5mm setup would work for 10 months of the year along the East Coast of Australia. In the other months you could layer up. A 3/3mm setup would leave you cold in winter and hot in February.
Fit, construction and getting the decision right
A bad fit for a 5mm wetsuit can feel colder than a poor fit for a 3mm wetsuit. Check how a wetsuit fits around the neck, wrists and lower back before you commit to a spearfishing wetsuit thickness, because a well sealed suit will outperform an extra millimetre of neoprene every time.
- Neck seal contact, with no gapping when the head turns.
- Torso length, so the suit does not pull off the shoulders when reaching overhead.
- Wrist and ankle closure, ideally smoothskin against bare skin.
- Open cell versus lined interior, with open cell measurably warmer for the same thickness.
The use of a 2-piece thicker wetsuit system is not a bad idea. If in doubt, always opt for the thicker wetsuit model. Cold ends are by far the biggest single cause of dive failure.