Not really. A patrol vessel is defined by role, not by size. The RoDN's Thetis-class patrol vessels are the size of frigates and sometimes referred to as patrol frigates, with a displacement of ~3,500 tons, a length of 112 m and a beam of 14 m. The armament however whilst possibly heavy for a patrol vessel considering they have a 76 mm/62 cal. gun, is well under what most frigates would be fitted out with.
Size is potentially a useful measure to distinguish between a frigate and a corvette, but that does not really carry over into vessels intended primarily for patrolling roles. An OPV and a corvette could very well be of approximately the same size, but would intended for very different roles with a correspondingly different fitout, as the role demands different capabilities.
Once a vessel starts to get below a certain size and displacement, then they become much less suitable for transiting long distances and/or crossing blue water/open ocean. The low end of OPV sizes (~80 m long, ~1,500 tons +/-) is right around the minimum for some of these requirements. If a vessel were to try and operate whilst being even smaller or lower displacement, then the vessel and crew could run into serious difficulties and risk, particularly if they had to weather a storm at sea and no port or harbour within a safe distance. What an OPV will usually do with the available space and displacement (beyond the DWT of a vessel) is to have victuals and bunkerage enabling long distance ops of corresponding duration.
A corvette OTOH would typically have much of the space and displacement used to fit weapons, combat systems, sensors and gen sets. As a result, both the transit distances as well as vessel endurance is typically lower for a corvette of a given size when compared with an OPV of similar size and displacement.
What this in turn means, is that if a number of the planned Arafura-class OPV's are cancelled in favour of some hypothetical corvette of a similar size and displacement, to also primarily operate in a patrol/constab role, then these hypothetical corvettes would be limited to much shorter endurance missions, and would also need to stick much closer to port. In that essence, they (the corvettes) might find themselves only able to operate in areas where the Attack-class or Fremantle-class patrol boats operated because they just could not have sufficient food or fuel loaded.
One of the other things is that typically an OPV, being built for a patrolling rather than a combat role, will usually be significantly less expensive to have built than a warship like a corvette or frigate of comparable size would be. Part of this is due to a much simpler weapons fitout, but also due to a less comprehensive sensor and CMS fitout. As an aside, I seem to recall that the electronics fiout to a modern warship is typically a significant portion of the initial acquisition costs, to the tune of a third to half the initial cost. Now IIRC the CMS and combat data systems fitted to the Arafura-class OPV, the SaabTech 9LV, are more than is normally fitted to an OPV, and likely put the cost of the RAN OPV's higher (I seem to recall a figure of AUD$300 mil. per vessel in 2018 dollars). This is a bit more than the ~NZD$110 mil. per vessel in 2010 dollars for the RNZN's OPVs. However, this figure is still well below the FY2017 cost of €400 mil. for the Braunschweig-class corvettes, which would otherwise have cost something like AUD$660 mil.
IMO it would be much better for Australia to keep with the Arafura-class OPV to meet patrolling needs, and get a proper class of frigates or destroyers to meet the need for potential combatants. To try and have a small vessel that can do both would IMO be a decision to spend too much money (and time, a key factor as well) to get another call of vessel that can do either patrolling or combat, but would perform those roles poorly.
