Riordan's Shoal lies further offshore than the better-known Black Head reef system north of the Richmond River mouth, and like most of the reef country around Ballina it is dived almost exclusively by private boat given the town's lack of a resident dive centre. The shoal rises from a sand bottom to around 12-15 m at its shallowest, with the surrounding ledges and gutters deepening to 25-28 m, putting the site's best features within Advanced Open Water range rather than the shallower depths typical of inshore Far North Coast reefs. As one of the more distant reef systems in the area, Riordan's Shoal sees a fraction of the diver traffic of the closer Black Head reefs, and local divers who do make the trip report the same seasonal chance of grey nurse sharks found up and down this stretch of coast between the Tweed and Solitary Islands aggregation grounds, along with kingfish, mulloway and general reef fish species holding on the structure in the current.

The shoal's depth and distance from shore mean it fishes and dives more like an offshore pinnacle than a simple inshore reef. Getting to Riordan's Shoal means first crossing the Ballina bar, which demands respect in anything but settled conditions and a favourable tide, and then a longer run than the inshore reefs require, so a trip here is very much a considered, weather-dependent undertaking rather than a casual outing. Documentation of the site's precise boundaries and depth range is limited to local and club knowledge, and prospective divers should treat the details here as indicative pending direct confirmation from someone with current local experience.

Ballina has no permanent full-service dive centre, so diving here is mostly private-boat or club-organised. Boats launch at the Ballina Fishermen's Co-op ramp or the public ramp on the Richmond River, then cross the Ballina bar — a notoriously changeable bar that should only be attempted by those experienced with it, in settled conditions and a rising or high tide. Riordan's Shoal lies further offshore than the Black Head reefs, requiring a longer, more weather-dependent run once past the bar.

This stretch of coast is exposed to easterly and south-easterly swell year-round, and the Ballina bar itself can close to small boats in a moderate swell or on an outgoing tide, so a trip depends as much on the bar as on conditions at the reef. Visibility degrades for days after Richmond River floods. Winter gives the clearest water and the best chance of grey nurse sharks.

Descend onto the shoal top around 12-15 m, work the deeper ledges to 25-28 m looking for grey nurse sharks and kingfish, and ascend along the shoal for the safety stop given the current.

No dedicated charter; dived occasionally by private boats and local clubs from Ballina when conditions and interest align.