The Problem with "Just Get Two NBN Connections"
The instinct is understandable. Your NBN drops out, you lose connectivity, someone suggests getting a second connection. Two connections — surely if one fails the other keeps you going?
Sometimes. But there's a critical factor that determines whether two connections are genuinely redundant or just expensive insurance that fails at the same time as the primary.
The factor is physical path diversity.
If both your NBN connections run through the same pit in the street, the same fibre node, or the same exchange — and that infrastructure fails — both connections go down simultaneously. This is more common than most people assume. A single excavation accident, a flooded pit, or a failed node can take out every NBN connection that runs through it, regardless of which RSP (retail service provider) they come from.
Optus NBN and Telstra NBN both run over the same nbn Co infrastructure. If the physical infrastructure serving your building fails, both connections fail. The provider name on the bill doesn't change the physical path.
What True Internet Redundancy Looks Like
True redundancy requires that the two connections use physically separate infrastructure. There are a few ways to achieve this:
The NBN runs on fixed-line infrastructure. A 4G or 5G connection runs on completely separate mobile tower infrastructure. These two physical paths have very few common failure points — a problem with the pit outside your building doesn't affect your mobile signal, and a mobile tower outage doesn't affect your fixed line. This combination gives genuine path diversity at reasonable cost.
If your building has access to a dedicated fibre service (not NBN) — from providers like Vocus, Aussie Broadband Business, or TPG Business — this can provide a second physically diverse path. Dedicated fibre typically comes with an SLA and guaranteed throughput, unlike NBN. More expensive, but justified for businesses where downtime is genuinely costly.
In larger buildings with multiple physical entry points (common in Melbourne CBD offices across multiple floors), it's sometimes possible to terminate two NBN connections on different physical paths. This requires confirmation from nbn Co that the paths are genuinely diverse — don't assume they are.
4G vs 5G Failover: Does It Matter Which?
For most Melbourne SMBs, 4G failover is sufficient. Here's the practical comparison:
| Metric | 4G LTE failover | 5G failover | Notes |
|---|---|---|---|
| Typical download speed | 20–80 Mbps | 100–500 Mbps | Varies significantly by tower load and proximity |
| Upload speed | 5–25 Mbps | 20–100 Mbps | Upload matters for Teams calls and SharePoint sync |
| Latency | 30–70ms | 15–40ms | Both are acceptable for Teams; 5G is noticeably better |
| Monthly data cost (20GB) | $40–60 | $60–90 | Failover-only usage is typically low |
| Coverage in Melbourne CBD | Excellent | Good — expanding | 5G coverage patchy in some suburbs as of 2025 |
| Hardware cost (SIM router) | $150–400 | $300–800 | Or integrated in FortiGate models with built-in modem |
The practical answer: if 4G coverage is strong at your location and your failover is genuinely just a backup (not load-shared with primary), 4G is sufficient and cheaper. If you're in a 5G coverage area and want the failover to comfortably handle full office load — not just email and urgent Teams calls — 5G is worth the extra cost.
Active-Passive vs Active-Active: How to Use Both Connections
Once you have two connections, there are two ways to use them:
| Mode | How it works | Pros | Cons |
|---|---|---|---|
| Active-Passive (failover only) | Primary link carries all traffic. Backup sits idle until primary fails, then takes over automatically. | Simple. Backup data costs are low (minimal usage). | You're paying for a second connection that's mostly unused. No performance benefit during normal operation. |
| Active-Active (load balancing) | Both links carry traffic simultaneously. SD-WAN routes by application type and link quality. | Both connections earn their cost. Better overall throughput. Faster failover (already monitoring both). | Requires SD-WAN configuration. Some applications don't tolerate asymmetric routing without careful setup. |
Our typical recommendation for Melbourne SMBs: active-active with application-aware routing. Teams calls go via the lowest-latency link, bulk traffic goes via the higher-bandwidth link, and if either degrades, the other picks up immediately. With a FortiGate and a modest 4G data plan (20–50GB per month), the additional monthly cost is $40–80 — easily justified by the reduction in downtime.
In active-passive mode, a 4G SIM used purely for failover typically consumes 1–5GB per month (monitoring traffic and occasional brief outages). In active-active mode, you're looking at 10–30GB per month for a typical 20-person office where the 4G link handles Teams calls and lighter traffic. A 20–50GB plan is usually more than sufficient.
The Automatic Failover Question: How Fast Is It?
Without SD-WAN or proper failover configuration, the answer is "however long it takes someone to notice and manually switch" — which could be 20 minutes, could be hours.
With a properly configured FortiGate SD-WAN setup:
- Health checks run every 2–5 seconds
- A link is declared failed after 2–3 consecutive failed checks
- Traffic is automatically rerouted within 10–30 seconds of failure detection
- When the primary recovers, traffic can automatically return (or stay on backup if you prefer manual switchback)
For a Teams call in progress when the primary fails: the call may momentarily drop and reconnect, or Teams may seamlessly continue over the new path, depending on timing. Most users experience a brief pause rather than a full disconnection.
What to Ask When Evaluating Your Setup
- Do your two connections use physically different infrastructure? If both are NBN, ask your MSP or ISP whether they share the same node or exchange. If they do, you don't have true redundancy.
- Is failover automatic or manual? If someone needs to plug in a different cable or change settings to switch to backup, that's not adequate for business continuity.
- How long does automatic failover actually take? Test it — unplug the primary and time it. If it takes more than 60 seconds, the configuration needs tuning.
- What is the backup connection's actual throughput under load? A 4G connection that delivers 40Mbps in a speed test at 6am might deliver 8Mbps during business hours when the tower is congested. Test it at the same time you'd need it.
The Bottom Line
For most Melbourne SMBs, the right answer is NBN primary + 4G/5G failover, configured for active-active with SD-WAN on a FortiGate. It costs less than you'd expect, delivers genuine redundancy, and means an internet outage is a minor inconvenience rather than a business stoppage.
If you're not sure whether your current setup delivers true redundancy, we can test it. We'll check the physical path diversity, test the failover timing, and verify that your configuration actually works the way you think it does.