This article doesn't answer many questions (because of course it doesn't - it's PTV), but V/Lines page does answer some of the initial questions I had. https://www.vline.com.au/News-Alerts/News-Articles/2023/December/Notice-of-industrial-action

Things I found notable:

Trains will progressively return from 8am but delays and cancellations will continue across the day, and services may be busier than usual while we work to recover the timetable.

So it appears that the first trains will not be running until 8, rather than 7. Although it's possible this is either a typo, or they haven't figured out their replacement paths.

Freight trains will continue to operate during the planned industrial action so always obey warning signs at level crossings.

A very limited coach replacement service will run on each line for essential journeys.

Coaches will not stop at any metropolitan stations.

Delays of up to 120 minutes or more are expected, and a temporary timetable will be available closer to the day for those passengers who need to travel.

Geelong and Warrnambool lines – a very limited number of coaches will run between Southern Cross, Geelong and Warrnambool. Coaches will not stop at Footscray, Sunshine and Deer Park.

Deer Park and Ardeer stations - a bus will run to Sunshine so passengers can travel on a Metro Trains service to and from the city. Buses will also run to Tarneit and Caroline Springs so passengers can connect to services on the Geelong and Ballarat lines.

Wyndham Vale Station – buses will run between Wyndham Vale and Werribee to connect passengers with Metro services on the Werribee Line.

Tarneit Station – buses will run between Tarneit and Laverton to connect passengers with Metro services on the Werribee Line.

Ballarat, Ararat and Maryborough lines – a very limited number of coaches will run between Southern Cross, Ballarat, Ararat and Maryborough. Coaches will not stop at Footscray, Sunshine, Deer Park and Ardeer.

Bendigo, Echuca and Swan Hill lines - a very limited number of coaches will run between Bendigo, Echuca and Swan Hill and Watergardens. Passengers will need to transfer to or from a Metro Trains service to continue their journey to and from Southern Cross. Coaches will not stop at Sunbury Station.

Seymour and Shepparton lines - a very limited number of coaches will run between Southern Cross, Seymour and Shepparton. Coaches will not stop at Craigieburn, Broadmeadows, Essendon and North Melbourne.

Albury Line - a very limited number of coaches will run between Southern Cross and Albury. Coaches will not stop at Craigieburn, Broadmeadows and North Melbourne.

Traralgon and Bairnsdale lines - a very limited number of coaches will between Bairnsdale, Traralgon and Pakenham. Passengers will need to transfer to or from a Metro Trains service to complete their journey to and from Southern Cross.

So the coach services will only replace certain train services, and as you can probably see, will be "very limited"

Temporary timtables will be published on this website on Tuesday 12 December as well as the mobile app and social media so please check for updates before you travel.

When they publish it, I'll post a follow up, but it sounds like the buses will probably be very few and far between, and considering it's the morning peak, probably very packed. If you have the choice, consider going where you need to go later in the day, or getting a lift to a metro station