Is it private work or ADF ?
Fishing land based around Darwin is not as easy as it sounds. Huge tides make it difficult.
If it is with the ADF. Try around the boat ramp in Larrakeyah barracks (or you might know someone in the ADF that can sign you in).
Having said that...do Not fish near any other boat ramps as the crocs often wait around the ramps for scraps.
The Gardens right up to Nightcliff can be good. Just remember the tides cos you can easy get stuck on a sand bank and swept away.
Buffalo Creek is a hot spot. But watch out for crocs.
A ferry ride to Mandorah is worth it. Take a tent and camp out the back of the pub (if they still allow it) and make it an over-nighter. However, do NOT fish there at night. The jetty is OK but the crocs are known to wait on the beach for drunks from the pub at night.
How far from Darwin are you prepared to travel? Manton Dam (south of Darwin) is fresh water. It is very safe. Not easy finding fish there without a boat, but well worth a day trip. It is stocked with barra and saratoga.
Shady Camp, to the east is another real hot spot. Do NOT fish south of the weir...the crocs will eat you. But a 4WD would be good for Shady Camp.
If you are restricted to the Darwin area, really your best option is to hire a guide or book onto a charter. Not cheap this time of year, the dry is the peak period. If your boat skills are up to scratch, you could try and hire a boat. Just remember when the tide changes, you need to have some pretty spot on boat skills...I am pretty calm on the water but Darwin Harbour has scared me more than once
itsaboat mate...Life is just a boat and then ya marry one !