What you are referring to is a radiant heater which works by mainly infra-red heat on a similar principle to bathroom heater lamps - without the bright light. These heaters do not directly heat the air, but heat up objects the ‘light’ falls on. Therefore, these heaters are perfect for open spaces - not necessarily open to the weather though (they can be subject to some rain though). Gas heaters work mainly by heating the air, so a lot of heat just goes into the atmosphere and not warming people. But remember, if you are sitting at a table, radiant heaters are above so you may end up with cold legs?
The radiant heat strip do come in various sizes including the 2400W sold by Bunnings. The restriction is the capacity of the power circuit - they should NOT be connected to a lighting circuit. If you choose a 2400W heater you would be limited to connecting only two to a power circuit - assuming you have a modern house with 20-amp circuit breakers, do not try this on old houses with rewirable fuses, it is just too risky. Do not consider the 3200W version as you will need at least a 15-amp power point and there are restrictions on what else is connected to that circuit.
These heaters are meant more for restaurants and really upmarket houses rather than the average alfresco, but if the e-Bay cost is a lot cheaper than Bunnings then it may be OK. An option is a portable quartz heater which are rated at 2000W, so still limited to two per power circuit, but they are a lot cheaper! These do have a wall mount option as well but you need to be careful they installed clear of anything flammable.
Just remember that if you do not have any handy power points to connect to you will need an electrician to install them. Do not try to use an extension cord as it could be dangerous - apart from a trip hazard they do get very hot causing a fire hazard.
Hope this helps.