I wouldn’t be relying on what the owners say their plumbers say.   Gas heaters can be extremely dangerous if not installed or vented correctly.  Is it the present situation where some of the flues may be leading into a non-vented roof space, presumably common property?   This definitely wouldn’t be in accordance with the standards, the flues should be vented outside the building.  If I were you, I’d be getting an expert in ASAP to inspect the installations and give you a report on what is compliant and any rectifications which need to be made. 

tiamaria – as DaveB suggested, your Owners Corporation (O/C) should obtain an expert report about the compliance or otherwise of the existing hotwater heaters installed at the Plan, and about current requirements for the various types of internally installed gas hotwater heaters.

I don’t know if that’s something that Integrated Building Consultancy may undertake as a wide-ranging investigation, but they’re one of FlatChat’s sponsors and you can contact them in the first instance on (02) 8814 5999. 

Secondly and if you want to keep the scope of the investigation specific to the fluing, any competent gas plumber would be aware of the relevant Australian Standard; and

Thirdly, if your O/C wishes to purchase and interpret its own copy the relevant guidelines then contact Sai Global on 131 242 and ask for a CD of AS 5601-2002 (AG 601-2002); they’re the authorised distributor for Standards Australia; and

Lastly, have a look at one gas hotwater heater manufacturer’s interpretive information HERE.