You have a blocked flue! Most likely from un-seasoned wood. If the wood has too much moisture it will smoke and leave creosote deposits that will build up and close off the flue, with results like yours.
Your flue needs to be at least 4m long to create a draft.
If your flue runs too cool, this allows gases (smoke) to condense & form creosote. This can happen from trying an overnight burn (too low) allowing top section of flue to cool & condense.
A good solution then is to change your flue to an insulated one. Twin skin stainless steel, 150mm inner, 200mm outer, has 25mm of packed Rock Wool surrounding & insulating the full length of the flue. You can even have an insulated inspection port at the bottom section allowing a flue brush in, to clean the full length from the top of the heater up, from the room. Far easier than removing firebricks and shelf from the firebox.
For info look up www.timelessclassicwoodstoves.com.au located in S.A.

If you are new to this heater there are some great Utube clips showing how to start these heaters. When first starting a cold heater, I have the air fully open and put the latch/ handle down and close the door against it. This leaves the door open about 6mm and provided more air. Once the fire is going well (5 minutes) and more fuel and shut the door.
Hope this helps

On a previous heater, we had the same problem and it turned out to be a build-up of ash on the baffle plate, we had burnt a lot of less than optimally dry wood the previous season, the secondary burn did not work as well and the unburned particles built up and obstructed the entry of smoke into the flue. A relatively quick fix - remove the baffle and get rid of built-up ash. I pays to service your heater at the start of every season.

Its 460mm long x 250mm high. Its small for the amount of energy produced due to its high efficiency. Cheers

Measures 44cm x 25cm = 1100 sq cm. Door is notably higher for easier loading plus, better viewing. Get the optional fan as its worth it for a large area. The heater is so well designed for USA & Canada with emissions requirements. It will burn o'night for up to 12 hrs. Consider an insulated SS flue to stay hot and minimise creosote build-up o'night. (see in Ebay for S.A. supplier)

1Linrob and Ken H, Thanks for the answer to my query. I was a lot keen on this fireplace except now feeling that the viewing area is smaller compared to other fireplaces. I have not had a look at this fireplace in person in any fireplace store/showroom. I might go for this one if I have a look in person and feel that the viewing area is Ok. Thanks again for your input

Maybe if it's 10Kw! The Neo2.5 has the inbuilt 'smarts' to burn 12 hours o'night. They did the research, not 'jamming open' the air control with a rivet or screw to comply with current Australian Emissions Regulations. Engineered for a climate down to -40 degrees Celsius, the Canadians know how to build an excellent wood heater. Do get the optional fan, it's worth it! We are in country Victoria, in the wheat belt and it gets very cold.
We have found it's frugal with quality wood and takes the chill off all of our 4 x bedroom home.

It is very cold in Canada for sure, but the houses are built to take it. Triple glazed windows, heavy duty construction, often double brick with a large insulated air gaps. A Canadian friend said no Australian house he had seen while here would even come close to meeting their standards. Different climate I guess... I dont understand your comment about 10KW. The old Port Philip we have is rated above this . I asked my question because I like the Neo, but could find no KW rating for it, only a vague "promise" it would heat between 280 and 300 square meters. Real world experience is better than figures in most cases anyway.

I’m sorry but I don’t know the heater your referring to. The 2.5 heats very well once going. I need to be careful in my 180m2 home as it can get to hot! I’ve learnt now how best to run it (at about 1/2 throttle) where it’s most efficient.
The fan does make a huge difference, so if you want to heat big spaces get that.
By the way my fan use to vibrate. So I chocked it with some wooden blocks and solved the problem.