Which engine do you have?
What grade fuel are you using?
What's the mileage and is it burning any oil?
When were the fault codes last read and cleared?

 

1.2-liter 3-cylinder petrol engine and i'm using high performance 5W-30 oil,
it's a 2006 Vauxhall

 

They don't do a 1.2 3 cylinder?? Its either a 1.0 3 cylinder or the 1.2 4 cylinder.
As for the oil as you are in a warmer climate I would go For a 10w40 or 20w50 oil depending what part of Australia you are in? Most of it is warmer than the UK and there you only need the thin oils in winter really.

Which ever size engine you have it will be I assume a "twinport" with a timing chain. It could be a number of faults. It's quite common for the coil packs to fail and as they do the engine losses power, for the twinport activation valve to stick, the rail link to break and this will cause power loss (you normally hear some kind of clicking sound from the back of the engine just below the cam cover area), the MAF sensor to fail, timing chain to be stretched and skip a tooth (usually they are quite rattly first), also for the throttle pedal and or wiring to play up, there is a plug in the drivers side kick panel with the junction between the pedal loom and the dash loom, check here for poor connections or corrosion on the pins.

My first step would be to use a diagnostic tool to scan for fault codes and read the live data from the car if you don't have any strange sounds from the engine that is?

How is it starting both hot and cold? does it ever stall easy? If so it may be the crankshaft sensor which is next to the starter motor and access is from under the car.

I would also recommend trying a high octane fuel I think from another post recently you guy get a 91ron, 95ron and 98ron fuel? The 98 is much better for the engine and back home we have a 99ron which is what I use and it returns much better MPGs, like 5 MPGs more than the 95ron so even though it costs more per litre it works out less £/mile over all.

Is it burning any oil? Normally they will drop about 0.5L in 5000miles. Any more and you have a fault, valve stem seals or worn piston rings. This can lead to a blocked catalyst and reduced power.