The decile ratings just indicate the socio-economic position of the community the school services and is used for funding by the govt.
You can have high decile schools that don't perform as well as lower decile ones. Though usually the decile rating and the performance are fairly tied as the amount of money the school has at it's disposal helps but some great teachers can be found in lower decile schools.
The only way really is to read the ERO reports for the school as these will give you an insight into what the school excels at and what it needs to work on. Some schools for example cater better for gifted students and/or students with learning problems than other ones do. Once you've read the report you can find a good match to your needs.
Hope it helps (mum's a teacher

). Also we're in Taka and we'll probably be sending Rowan to Hauraki Primary or St Joseph's.