Senior Engineer (Lessn)

About Lessn Lessn lets a business pay any supplier invoice or recipient on a credit card, even suppliers who only accept bank transfers. The supplier receives a normal bank transfer the next business day, the fastest payout in the category, while our customers keep up to 55 days of working capital and earn rewards points on money that was already going out the door. Lessn reconciles automatically back into Xero and MYOB, with further accounting integrations on the roadmap. Under the hood, Lessn is an orchestration engine sitting between the card networks, banks, the acquirer and the payout rails. We are now building the next layer: an agentic and deterministic AI/ML engine that decides the optimal way to pay every invoice, automatically, per business, using live and holistic open banking data. Nobody else in Australian B2B payments is building this, and it is a core part of what the next engineering hires are here to build. Lessn is live and processing for 130+ active Australian businesses across professional services, construction, hospitality and retail, with tens of thousands of invoices paid and chargebacks running at roughly 0.02% of turnover, an exceptionally clean book for a B2B card business. We are AUSTRAC-enrolled with a full AML/CTF programme that has passed institutional review, and our direct American Express accreditation is well progressed. The team you are joining Lessn is a sub-15-person company led by David Grossman (Founder & CEO), a fourth-time founder, and Simon Daniels (COO & AML/CTF Compliance Officer). The board and investor base includes senior operators from Bain, PayPal ANZ, Afterpay, REA Group and Canaccord Genuity. The Lessn platform was originally built by an outsourced development partner. In 2026, we brought the platform in-house and engaged a new engineering partner while building our permanent team. These two roles form that team. You will not inherit someone else’s standards; you will help set them. That is both the opportunity and the work. Our culture We operate like a small, high-trust unit: everyone delivers, everyone has a voice in product and operations, and nobody hides behind process or hierarchy. You'll work in our office in Circular Quay, Monday, Wednesday and Thursday, and from home Tuesday and Friday. Role summary We are hiring a Senior Software Engineer to join Lessn's embedded engineering team. You'll build production systems across our payments orchestration platform and contribute to the agentic AI/ML layer that decides the optimal way to pay every invoice. You'll also mentor our graduate engineer, holding the bar on code quality and supporting their growth. Your first twelve months Months 1–3. Finish and stabilise the platform migration (supported by the previous outsourced partner), close out required compliance work, and take full ownership of deployment. Much of this is inherited work. It is not glamorous, but it is the foundation for everything after it. Months 4–6. Automated tests run on every commit and block a bad merge. Deploys go through a pipeline, not by hand, and can be rolled back. A non-production environment can be rebuilt from code. Supplier verification and KYB automation are live. The graduate is shipping reviewed work without you unblocking them daily. Months 7–12. The data foundation and the first working version of the decision engine. This is where the product becomes genuinely defensible, and you will have built it. The practical detail Production support. We run live payment rails. You are part of a shared escalation roster and are the technical escalation point for customer-impacting incidents. It is not a heavy on-call load, but it is real. Regulated environment. You will complete AML/CTF training on joining and annually and follow our data handling and change control obligations. Appointment is subject to a national police check. Performance Reviewed at. 3 months, 6 months, then annually, against the outcomes set out above. Responsibilities Software engineering: Design, build and deploy production software across Lessn's payments orchestration platform, including integrations with card networks, banks, the acquirer and payout rails. Data and AI engineering: Contribute to the design and delivery of the AI/ML decision engine that optimises how every invoice is paid (cheapest, most points, longest float, best use of cash, early-payment discount), using real-time open banking data. No prior AI/ML experience is required, we will teach you. Accounting integrations: Build and maintain reconciliation integrations with Xero, MYOB and future ERP platforms. Engineering process: Introduce and champion repeatable, scalable engineering practices, including CI/CD, automated testing and infrastructure-as-code. Mentorship: Mentor our graduate engineer. Review their code, support their development, and hold the quality bar on what ships. Quality: You are a senior line of defence before work reaches production in a regulated fintech environment. If it is not right, it does not go. Roadmap input: Feed insights from delivery back into the product roadmap, across vertical integrations, accounting connectors, the Payday Super module, multi-currency payments and real-time reconciliation. What You BringRequired Experience 5+ years hands-on software engineering experience, including production systems in a commercial environment. Strong command of Python, and experience with at least one strongly typed language (C#, Java, C++ or similar). Experience with cloud infrastructure (AWS, Azure or GCP), infrastructure-as-code and containerisation. Demonstrated ability to design and ship software independently, with minimal oversight. Comfortable operating in a small team with direct exposure to founders and rapid decision-making. Preferred Experience Experience in fintech, payments or another regulated industry. Exposure to AI/ML engineering and the data pipelines that feed it (not required, we will teach you). Startup or scaleup experience in a fast-paced technology business. Experience mentoring or leading junior engineers. Your traits Skills and tools can be learned. These cannot: Curiosity that drives you to build, experiment and learn without being asked. Commercial pragmatism that solves the customer's problem, not the most interesting one. Judgment that lets you pick an approach and move without perfect information. Directness that speaks the truth without ego getting in the way. Genuine satisfaction from making someone else better, not just shipping your own code.