Seeing kangaroos in the outdoor is generally the top thing to do for visitors coming to Australia. Here's my list of top 4 places I have almost always spotted kangaroos.
1. Morisset - Silky Oak Drive This one is my top spot for kangaroo spotting. Only 120 kilometers north of Sydney, you will never go away from here without sighting kangaroos. And surprisingly there are plenty of them and they aren't shy to take a selfie with you.
Drive up to Morisset and navigate towards the open plains along Silky Oak Drive near the Morisset Psychiatric Services Hospital. You'll find that there are plenty towards sunset.

Kangaroo spotting at Morisset

Kangaroos at Morisset
2. Hunter Valley Hunter Valley is a great place for kangaroo spotting at dusk (or sometimes during the day too). Driving up to the vineyards, you always spot kangaroos in the valley or just hopping by in the distance. Watch out for the 'Kangaroo Ahead' signs and you will most definitely find them there. We spotted a group of around 80-100 at dusk but they ran away too quick for us to click.

Kangaroos at the Hunter Valley
3. Sanctuary point (near Hyams beach)On a trip to Hyams beach, we stayed over at Sanctuary point only to find kangaroos in the neighbour's backyard. Sanctuary point is around 200 kms south of Sydney

Kangaroos in Sanctuary point
Later on while driving to Hyams beach we saw plenty of them en-route. None of them stayed to pose for us though.
4. Jenolan caves On the way to Jenolan caves from the Blue Mountains, we found many kangaroos. The route is narrow and forces you to go slow and gives the passengers some time to look around for the kangaroos hopping about.
I'd love for readers to share any more places around Sydney that they have spotted kangaroos.