Auckland council provides free of charge a Geographic Information System (GIS) which is a valuable tool on the web showing Auckland in a geographic format - http://maps.aucklandcouncil.govt.nz/aucklandcouncilviewer/
The website provides a wealth of free information that will help you determine important aspects about a sites viability when making your initial assessments.
I consider the proximity for all main services needed to enable a site and ultimately dwellings to function. These include stormwater run off, wastewater, potable water, electricity, gas and telecommunications.
Wastewater lines are shown in red, stormwater lines in green, potable water in blue. It is important to analyse your site and determine how you will get these services to your site if they are not already serving it. Brownfield sites (existing use) will have services already available, but when increasing the number of units on the proposed location, additional/increased upgrading will almost certainly be required. You need to figure out how to get the recognised public lines in to serve your increased requirements. An ideal scenario is where the necessary lines are already passing through your site or within a very near proximity. One also needs to look at capacity. A good point to discuss with your civil engineer about loadings.
More common is working out how to get the necessary services to your site which would typically require extending the line and/or connecting via a neighbouring (private) property. Both can be costly and time consuming. For the latter, neighbour's consent is required and can often require some financial contribution for their inconvenience and disruption. If that fails, there is the Local Government Act which albeit a longer process, can obligate the relevant neighbour to permit you to make the necessary connections.
It's quite common for a site to be unviable purely because services aren't able to be provided to the site economically and thereby crippling the financial viability of the project.
Also the site topology is an important factor, as these services require sufficient 'fall' to enable the stormwater and wastewater to fall sufficiently in order not to block / cause disruption to the overall network.
Here is a screenshot of the GIS system made available by the Auckland City Council...
The GIS illustrates the land topology. The topological lines are shown in 0.5 meter levels, so it is a relatively straightforward exercise to study the lay of the land. This will aid your determination of important considerations, notably how the development might look and also looking at how you will get services to your site. Specific planning development controls can also be determined to a certain extent.
Flooding is an important issue to consider. Council will look at the risk of your proposed site in relation to flooding risk. The main risks are termed Flood Prone, Overland Flow Path, Flood Sensitive . These are events that are typically measured against a 1 in 100 year event and the council will want to know how you have mitigated this risk when this event occurs. These are typically not unassailable, just requiring good planning and strategies to address this. A good surveyor and geotechnical engineer can help you resolve this.
The Auckland Council GIS has a useful measurement tool which help measure site size and determine critical planning controls that would need to be taken into consideration depending on your specific site requirements. When analysing a site, one can actually perform and layout rudimentary site schemes, rights of way etc. All can be done at no cost prior to engaging a landl surveyor.
Note - This information above is my viewpoint alone and should not be construed as specific advice. I strongly advise that you seek professional advice from a qualified professional when conducting your own analysis.