May I know if one is to rent out a house in Australia, is it true that the landlord must pay a monthly allowance/commission to the agent in addition to the one time agent's commission when the house is rented out?

Thanks for your help in advance! =)

There are not too many musts between landlords and agents, for that is a business arrangement which is entirely negotiable between those parties and all I know of [with limited experience ] is that a % of rent is payable by landlord to an agent.
There could be a situation if it was a new business connection and the agent had suggested some advertising of the vacancy, that the landlord could have to make an additional one time payment but in most cases I'd suspect an agent to just do their inspection and add the property to their rentals list, there being a high demand in city areas.

But whatever occurs that is nothing for the tenant to be concerned with as properties will have a rental set and a tenant will pay a bond, usually four weeks rent and then weekly/fortnightly or monthly rent paid in advance and all that including an inspection report re premises condition on a lease agreement, leases usually a minimum of three or six months and then on a month by month basis though a tenant can request for longer minimum priod and then if wanting to request extensions in longer periods.

Make sure when you inspect any premises, you have the agents condition report and if you see any areas of what you may consider even minor damage, blemishes in paintwork, carpet wear/stains or cracked floor tiles, chipped woodwork and faulty flywire screens etc. that you record those, room by room, having the agent acknowledge by their agreement signature and get what you needed fixed.