Tapa Martinthi Yala

Like most of the options you head south east from Salt Pan Junction back towards Salisbury Highway. It would seem that this path is called Tapa Martinthi Yala, at least until it crosses the highway and then it's the Northern Connector Bikeway.
Anyway beware of the sharp turn at the end of the long drag and the small bridge soon after the turn as it still doesn't have a sealed surface.
The culvert under Salisbury Highway often has a layer of slime, sometimes water too. Only the right hand tunnel actually extends under the highway.
After the culvert there is a turn to the left that will take you to South Road but go straight ahead next to the wetland.
The path will eventually turn to the right and exit to Magazine Road but instead of heading for the road we'll go bush instead.

Greenfields Wetlands

Instead of continuing on the Northern Connector Bikeway and then taking the Gawler Greenway north take a shortcut through the wetlands. It's pretty much a single track of dirt with gravel off line and a number of small wooden bridges. A number of these bridges are around a bend of the track so they come up on you fast.
The track ends at a gate next to the Salisbury Highway and a bit further on it connects to the Gawler Greenway before reaching Kidman Road.

Railyards

Continue down Kidman Road, past the OTR and rather than turn onto Vater Street (which I assume is the path that the Gawler Greenway would currently follow) continue on Kidman Road. There is a sign about it being private property but I think that's just lazy signage by One Rail Australia and they should be on the roads to the right. At the end of the road turn to the left and take care on the one lane road under Port Wakefield Road.
Before this slip road enters Port Wakefield Road there is a unsealed driveway to the right. The gate used to be open but now there is a gap in the fence to the left of it. There are small cobbles around the gate area but it reverts to gravel/dirt soon enough. There is a gate blocking access to the actual rail lines but there is a dirt "lane" along the fence line.
The lane comes to a junction. I don't know if it is possible to continue along the fence line so bear to the left and cross the paddock to a gate at the end of Broadwater Crescent.

Mawson Lakes

It's a combination of on street and paths to get from Broadwater Crescent through to the Boat Deck Cafe.
Follow Broadwater Crescent and turn right onto Caswell Circuit and then right again onto Hindmarsh Circuit. Right again onto Shoalhaven Circuit. Just before Weatherstone Crescent you need to take to the footpath (either via a driveway or the pedestrian ramp) as this path will take you behind some houses. When you come to a small park (part of Elder Drive Reserve) there is a small bridge. Cross it and immediately turn right. Continue north east on MacMillan Avenue. Just before it bends to the left there is another gravel path to the right that you should take as this leads to the fine new shared path bridge over Dry Creek. After the bridge go straight ahead onto Ridley Street. to your right on Augustine Street is a pedestrian crossing leading to a ramp up to Elder Smith Road. There is a shared path as well as on road bike lanes on Elder Smith Road. Use the shared path to cross the railway line and use the ramp on the other side to descend down. Turn left an exit to the road. Go straight ahead and this will take you to the Dry Creek Trail. Follow the trail, crossing The Strand (carefully, the ramps are terrible). Exit the trail and turn left onto Mawson Lakes Boulevard. Continue through the roundabout and the cafe will be on your left.