Bethells beach is not as long as Muriwai, and therefore you do need to take care not to turn up during the most popular times in summer when the beach will be crowded (and so will the carpark). It is a bit of a trek, just to find that you have to turn around, and go home.
As with most beaches, low tide is when you want to aim for. The added benefit at Bethells is that during low tide you can ride an additional stretch of beach that is inaccessible at high tide.
Stay off the Dunes! No matter where you ride, you should always stay off the dunes; only ever ride on marked trails through dunes, or you will be damaging very sensitive ecosystems, and potentially causing damage to the the dunes themselves (and causing them to shift).
Be aware of nesting birds.
One big reason to always ride below the high tide mark is to ensure that you are not riding in areas used by nesting birds (birds don't nest under water, so if you are below the high tide mark you can be assured you are not destroying nests).
Whichever way you drive, you are going to have to tackle the Waitakere ranges, or at least a small portion of them. Even once you make it to the Bethells\TeHenga roads intersection, while the road flattens out somewhat, it is winding, and often narrow. Be aware of traffic building up behind you, and look for the one or two spots where you can safely lets others pass (Te Aute Ridge Rd is probably one).
Parking at Bethells is good, but there is no special horse park, so be considerate of others, and as said before, don't turn up when the place is heaving with beach goers. As with all public places, please pick up all horse poo, and either take them home, or put into the gardens (out of the way).
If it's your first time at Bethells, keep driving past the Lake Wainamu, or Te Henga Walkway parking areas (just after the one way bridge). There is a grassed area just (to the left) before the main carpark at the beach.
Both the main public parking area, and a grassed area just to the left (as you drive in) provide ample pull-through parking for horse floats and trucks.