Hi, my Ford Falcon AU3 has a very big oil leak, and not being an expert I'm not too sure what to do about it.

Background: it has done 215K km and was previously leaking a little oil (drop every 20s or so), dripping from the centre seam of the sump about halfway back.

It was just serviced yesterday. Mechanic pointed out a few issues including small leaks around front and rear seals; these haven't been repaired yet. Got home okay last night and did another 40-50 km this morning and now engine oil is just pouring out of it like a stream. It only leaks when the engine is running and, I think, the leak is slightly to the passenger side in line with the rear of the sump area (not 100% on that though). Is this likely to be one of the seals that's gone completely? Because my first instinct is that it could be the oil pressure sender or a faulty/poorly-fitted oil filter. All helpful advice greatly appreciated.

Rear Main Seal?

If its the rear main seal its a gearbox out repair.

All fixed up now - oil was running down the side of the sump about halfway back, and when I traced it back up it was squirting out of the oil pressure sender. Swapped that out, topped up the lost oil and it's good to go again. I think that was the source of the slow leak before too. Very happy it wasn't the rear seal. I was quoted a very unwelcome price for getting that replaced.