Thanks for all the comments.
Northern Farms is just past Fourways heading towards Pretoria, so about 20 mins away for me. It is a working farm with lots of cattle and some crops, and it is very popular with cyclists, offering lots of trails of varying difficulty. What's nice is that cyclists have to leave their cars at the car park, but if you are there for birding you are allowed to drive through the park. When I was there, there was only one other couple birdwatching, so only two cars on the roads.
Just a pity about all the cyclists
but there are some roads reserved for birders which the cyclists are not allowed to use!
One thing to note is that it is only open to the public on weekends and public holidays.
Diepsloot Nature Reserve
This nature reserve is one of Gauteng's best kept secrets, with around 270 recorded bird species.
Diepsloot Nature Reserve, which is about 1600 hectares in size, was proclaimed as a nature reserve in 1960 as a green belt around the Northern Wastewater Treatment Works.
The area was used for irrigation for treated effluent through a maturation pond system and has been farmed for a number of years with Bovelder cattle and various crops.
Today there is no longer a need to depend as heavily on the farmland as a wastewater treatment option and as a result other uses for the farm were considered, foremost among these being increased recreational use.
In the medium term portions of the farm will continue to be used for effluent treatment and farming will for the time being continue as before.
It is not a pristine area by any means, with artificial dams and stands of exotic trees and reedbeds being the dominant habitat type.
However, there are few wetlands of any significance in the general area and protecting and improving the nature reserve status is therefore of paramount importance and is something that the Northern Farm Recreational User Group Committee (NFRUG) regards as a priority.
In addition, Johannesburg Water has committed to creating a sustainable facility for integrated recreational use by members of the public. Today the part of the reserve that is most visited is the northern section, more commonly known as Northern Farm.
Birding
Early morning birding is recommended in summer. In winter a later arrival with a longer stay is recommended, while summers are generally hot at midday. Optimal times of the year for birding are autumn and early summer with the arrival or departure of the migrants.
The dams and reed areas provide excellent birding all year round. It is advisable to bring a scope with you, as several of the dams have visible mixed heronries, and raptor nests. These areas have high densities of the following birds: Southern Red Bishop, Southern Masked-Weaver, Egyptian Goose, Yellow-billed Duck, Spur-winged Goose, Cape Shoveler, Southern Pochard, Red-knobbed Coot, Common Moorhen, Reed and White-breasted Cormorant, and Little Grebe, to name some. The reed areas should produce Black Crake, African Purple Swamphen, Purple Heron, Little Bittern (both races), Goliath Heron, Common Waxbill, Orange-breasted Waxbill and a variety of warblers dependent on the season, such as Lesser Swamp-Warbler, Little Rush-Warbler, with Great Reed and Sedge Warbler frequently encountered.
The grassland and cattle kraal areas in summer often have Yellow Wagtail, Amur Falcon, Greater and Lesser Kestrel, African Pipit, Pied Starling, Cape Longclaw, Cape Grassbird and both Red-capped and Rufous-naped Lark. Nomadic birds like the Capped Wheatear are also seen regularly, and numbers of Long-tailed and White-winged Widowbirds can be expected.
Hirundines are well represented, and in summer up to 8 species can be found, including South African Cliff-Swallow, Common House Martin and Sand Martin.
The reserve is known for its raptors with Long-crested Eagle, Black and Ovambo Sparrowhawk breeding in the extensive poplar stands, while Black-chested Snake-Eagle and Lanner Falcon are also observed frequently. Specials such as Osprey, African Marsh Harrier, and Black Kite are also known to occur and European Honey-Buzzard is recorded annually. This is the only known locality in the Greater Johannesburg area where African Fish-Eagle regularly breeds.
The remnant woodland areas with Rhus and Acacia species are excellent for resident Fiscal Flycatcher as well as migratory Fairy Flycatcher. Willow Warbler, Cardinal Woodpecker, Arrow-marked Babbler, Green Wood-Hoopoe are commonly seen in the woodland areas as well, especially alongside the Jukskei River. Birds that are often seen in overlapping habitats include the Bronze Mannikin, Tawny-flanked Prinia, Common Waxbill, African Stonechat and Cape White-eye and Kurrichane Thrush.
Unfortunately I didn't find the Fish-eagles.
Source:
http://www.northernfarm.co.za/birding.html
I won't post them all as some pics are not great - just proof pics. 
